Tuesday, February 5, 2013
Bad Blood, Bad Writing
After watching the episode of Grey's Anatomy called 'Bad Blood', it is time to call out the writer, Jeannine Renshaw. Usually, ‘Grey's’ will leave well enough alone when it comes to laboratory medicine, but this episode crossed the line in so many ways. Basically it was about a trauma victim who was a Jehovah Witness with a living will that explicitly stated no blood products to be transfused. Of course one of the self righteous characters had to try and smuggle a unit of blood into ICU to transfuse to the patient, and was caught by the wise and all seeing Christine Yang.
How many transfusion medicine practitioners were appalled at this scene? Does the general public still accept the notion that blood and blood products float through the hospital setting to be used at the Doctor's whim? One couldn't help but notice that the unit in the scene had been pierced, so it was basically useless.
Couldn't the scene had been a bit more realistic, or at least a disclaimer at the bottom stating that in an actual hospital blood and blood products are diligently monitored? After all is television the medium that has those disclaimers in car commercials "driven by professional driver in a closed course"?
What is most frustrating is that after all the work that has been done to regain the public’s trust in transfusion medicine; one episode can blow it back into the Stone Age.
Friday, February 10, 2012
Twenty eight percent
It has been a while since the last posting. The goal this year was to have 52 posts (one/week) starting Sept. 19th. So far, 15 have been posted, a completion rate of 28.8%, a little of a quarter of a way done. The last one posted was on Dec. 28th,2011.
So why the delay?
Credit goes to Dave Crenshaw, author of “The Myth of Multitasking.” After reading this small, yet very informative book, one will look at multitasking as the fallacy it is. Explaining why would do the book and Mr. Crenshaw a disservice, break down and buy the book.
How does this affect the Labvocate?
Currently, the Labvocate is in the middle of a project that has to be done. The Labvocate was under the illusion that this project and writing posts at the same time could be accomplished. The reality is that neither occurred.
Multitasking is a myth.
Long live dedication to a single project till it is completed!
So why the delay?
Credit goes to Dave Crenshaw, author of “The Myth of Multitasking.” After reading this small, yet very informative book, one will look at multitasking as the fallacy it is. Explaining why would do the book and Mr. Crenshaw a disservice, break down and buy the book.
How does this affect the Labvocate?
Currently, the Labvocate is in the middle of a project that has to be done. The Labvocate was under the illusion that this project and writing posts at the same time could be accomplished. The reality is that neither occurred.
Multitasking is a myth.
Long live dedication to a single project till it is completed!
Thursday, December 29, 2011
RFID in Transfusion Medicine
Radio Frequency Identification (RFID), or as it is also known as, Dedicated Short Range Communication (DSRC), is a technology that helps track an animal, person, or object. It does this by using a radio signal to locate what it is searching for. Unlike a barcode, it does not require direct contact or light-of-sign scanning.
There are three components in a RFID system; an antenna, transceiver and a transponder. The antenna sends out a radio frequency wave that is picked up by the transponder which sends back a signal. An example could a warehouse that wants to keep track of how boxes are in it. Each box will have a tag on it that has a transponder on it. An antenna will send out a radio wave that will activate the transponder on the tag to send a signal back. The transceiver will then receive this signal, and then a computer will not only count how many boxes respond, but where each box is in the warehouse.
This technology is being used in Transfusion Medicine. Of all the disciplines in Medical Laboratory Science, Transfusion Medicine is the only one where the end result (a blood product) will be given to the patient. Sometimes a transfusion reaction can occur from the blood product can cause a reaction to the patient, some of the most serious as a result of improper collection, processing or administration.
In any laboratory procedure, it is broken up into three main stages, pre-analytical, analytical, and post analytical.
In the pre-analytical stage, it is the proper collection of the specimen. This stage relies on properly identifying the patient and collecting the right specimens. If a three unit crossmatch is ordered for Mr. John Smith in Room 309, collecting a specimen from Mr. Jon Smythe of Room 903 will cause more harm then good. If Mr. Smythe in Room 903 needs a CBC, collecting a throat swab would be useless.
How RFID helps in the pre-analytical stage is that the person collecting the specimen receives a Transfusion Request Form (TRF) which then produces a label with a tag on it. At the patient’s bedside, the phlebotomist uses an RFID reader that confirms their ID from a tag on their ID bracelet. After the sample is collected and the label is put on it, it will then be scanned generating an electronic signature and updating the status of the specimen collection. The specimen will then be brought to the lab for testing.
At the analytical stage, the specimens will be processed, and RFID will signal whether or not the blood products are compatible with the patient.
In the post analytical stage, when the RN transports the blood product, one of two things can happen once it is transported to the ward. One, it can be transfused immediately, or something occurs that postpone transfusion. What happens to that blood product, for example a unit of blood? Packed red cells cannot stay out of a fridge longer than 30 minutes before transfusion. If the RN can document that the unit is put in a suitable fridge on the ward within 30 minutes, the unit does not have to be returned to the lab, and it can be used later. RFID allows the RN this option. While RFID cannot measure temperature, it can measure that the unit is put in the fridge, and for how long.
If there is no delay, RFID can be used to confirm that the blood product is the correct one the patient is to receive. This is done by using the RFID reader to read the tag on the patient’s armband and the tag on the unit. A mismatch will sound an alarm and prevent a mistake.
RFID has potential cost savings, especially those associated with Risk Management. Each hospital will have to determine if their organization would benefit from this technology. Currently, this system is not being used by the Canadian Blood Services, although some hospitals in the U.S. are. There is the argument that common sense and following standard operating procedures negate any reason for using RFID.
But then again that was what they said about computers and barcodes 15 years ago.
There are three components in a RFID system; an antenna, transceiver and a transponder. The antenna sends out a radio frequency wave that is picked up by the transponder which sends back a signal. An example could a warehouse that wants to keep track of how boxes are in it. Each box will have a tag on it that has a transponder on it. An antenna will send out a radio wave that will activate the transponder on the tag to send a signal back. The transceiver will then receive this signal, and then a computer will not only count how many boxes respond, but where each box is in the warehouse.
This technology is being used in Transfusion Medicine. Of all the disciplines in Medical Laboratory Science, Transfusion Medicine is the only one where the end result (a blood product) will be given to the patient. Sometimes a transfusion reaction can occur from the blood product can cause a reaction to the patient, some of the most serious as a result of improper collection, processing or administration.
In any laboratory procedure, it is broken up into three main stages, pre-analytical, analytical, and post analytical.
In the pre-analytical stage, it is the proper collection of the specimen. This stage relies on properly identifying the patient and collecting the right specimens. If a three unit crossmatch is ordered for Mr. John Smith in Room 309, collecting a specimen from Mr. Jon Smythe of Room 903 will cause more harm then good. If Mr. Smythe in Room 903 needs a CBC, collecting a throat swab would be useless.
How RFID helps in the pre-analytical stage is that the person collecting the specimen receives a Transfusion Request Form (TRF) which then produces a label with a tag on it. At the patient’s bedside, the phlebotomist uses an RFID reader that confirms their ID from a tag on their ID bracelet. After the sample is collected and the label is put on it, it will then be scanned generating an electronic signature and updating the status of the specimen collection. The specimen will then be brought to the lab for testing.
At the analytical stage, the specimens will be processed, and RFID will signal whether or not the blood products are compatible with the patient.
In the post analytical stage, when the RN transports the blood product, one of two things can happen once it is transported to the ward. One, it can be transfused immediately, or something occurs that postpone transfusion. What happens to that blood product, for example a unit of blood? Packed red cells cannot stay out of a fridge longer than 30 minutes before transfusion. If the RN can document that the unit is put in a suitable fridge on the ward within 30 minutes, the unit does not have to be returned to the lab, and it can be used later. RFID allows the RN this option. While RFID cannot measure temperature, it can measure that the unit is put in the fridge, and for how long.
If there is no delay, RFID can be used to confirm that the blood product is the correct one the patient is to receive. This is done by using the RFID reader to read the tag on the patient’s armband and the tag on the unit. A mismatch will sound an alarm and prevent a mistake.
RFID has potential cost savings, especially those associated with Risk Management. Each hospital will have to determine if their organization would benefit from this technology. Currently, this system is not being used by the Canadian Blood Services, although some hospitals in the U.S. are. There is the argument that common sense and following standard operating procedures negate any reason for using RFID.
But then again that was what they said about computers and barcodes 15 years ago.
Wednesday, December 28, 2011
The Lowly Tape Measure-Newest Diagnostic Tool for Heart Disease
It has long been know that cardiovascular disease is the major cause of death in the developed world. Since 1948 with the groundbreaking study of the population of Framingham, Massachusetts, the risk factors of hypertension, elevated cholesterol and stress were identified.
A recent study, the Interheart study (http://www.medscape.com/viewarticle/489738) took the study of risk factors to an international level, including lower and middle income countries. Nine common factors were determined to contribute to heart disease; increased ApoB/apoA1 ratio, smoking, diabetes, hypertension, abdominal obesity, psychological factors, daily consumption of vegetables and fruit, exercise and alcohol intake.
So what’s the difference between obesity and abdominal obesity?
Abdominal obesity is defined as the accumulation of abdominal fat resulting in an increased waist size. For men it is defined as having a waist of more than 40 inches, 35 inches for women.
There you have it. Two diagnostic tests are required to determine your cardiovascular risk. One is a blood test that requires a blood specimen. The other involves a tape measure. Simply take the tape measure, wrap it around your waist at belly button limit and see what your risk factor is. Both are necessary, and will affect the other.
A recent study, the Interheart study (http://www.medscape.com/viewarticle/489738) took the study of risk factors to an international level, including lower and middle income countries. Nine common factors were determined to contribute to heart disease; increased ApoB/apoA1 ratio, smoking, diabetes, hypertension, abdominal obesity, psychological factors, daily consumption of vegetables and fruit, exercise and alcohol intake.
So what’s the difference between obesity and abdominal obesity?
Abdominal obesity is defined as the accumulation of abdominal fat resulting in an increased waist size. For men it is defined as having a waist of more than 40 inches, 35 inches for women.
There you have it. Two diagnostic tests are required to determine your cardiovascular risk. One is a blood test that requires a blood specimen. The other involves a tape measure. Simply take the tape measure, wrap it around your waist at belly button limit and see what your risk factor is. Both are necessary, and will affect the other.
Tuesday, December 27, 2011
MDR/XDT TB and Carbapenem
The ‘White Plague’, or as it is commonly known Tuberculosis (TB) is making a comeback in not only the rest of the world, but in North America as well. HIV is helping the spread, an immuno-compromised patient be more susceptible than a healthy person.
In 2009, it was estimated by WHO that there were 9.4 new cases of TB. 3.3 % of these new cases (approximately 250,000 patients) had Multi Drug Resistant (MDR) TB. The criteria for a TB strain to be classified as MDR is to be resistant to the traditional treatment of isoniazid and rimfampin. In 2008, MDR was estimated to have killed 150,000.
To make matters worse, an even more strain strain has emerged, Extensively Drug-Resistant (XDR) TB. This strain is not only resistant to isoniazid and rimfampin, it is also immune to a fluoroquinolone and resistant to injectable amikacin, kanamycin or capreomycin.
With the exception of fluoroquinolones, there hasn’t been any new anti-TB antibiotics introduced for treatment.
TB was one of the first organisms that penicillin was not able to treat. The causative organism of TB, Mycobacterium tuberculosis, naturally produces a chemical called beta-lactamase which inhibits the active ingredient of penicillin, beta lactam, from working.
Recently though, researchers (http://www.einstein.yu.edu/home/news.asp?id=305 ) found that by combing two different drugs together, XDR TB could be killed in a laboratory setting. These two drugs were Clavulanic acid and meropenem ( a Carbapenem) .
Clavulanic acid by itself has no therapeutic effect. What it does do is neutralize beta-lactamases produced by bacteria. Unable to destroy the beta-lactam of the antibiotic, drugs such as penicillin can destroy the bacteria.
When Clavulanic acid is combined with a meropenem (a beta-lactam antibiotic), they can effectively destroy TB in a lab setting, but can they be used in a clinically setting?
In 2010, Belgium doctors treating a 14-year-old girl from Chechnya with XDR-TB with answered that question. With no other options to treat the acutely ill and malnourished patient, Clavulanic acid and meropenem produced positive results in eleven weeks. (http://www.stoptb.org/news/stories/2011/ns11_035.asp)
This is indeed positive news, but some cold realities have to be faced.
First of all, these drugs have to be administered intravenously. That not only requires an infrastructure to be put into place to provide patients this service, there is also the risks associated with long-term I.V. therapy (ie infection).
Secondly, there is the cost of these drugs.
Thirdly, clinical studies will have to be done to prove it’s effectiveness.
Finally, there is the possibility that resistance may develop to this combination. After all meropenem has recently been found to b useless against New Delhi metallo-beta-lactamase-1 (NDM-1) an enzyme at produces undetectable resistance to Beta-lactam drugs.
But hopefully this will prove to help with the global fight against TB.
In 2009, it was estimated by WHO that there were 9.4 new cases of TB. 3.3 % of these new cases (approximately 250,000 patients) had Multi Drug Resistant (MDR) TB. The criteria for a TB strain to be classified as MDR is to be resistant to the traditional treatment of isoniazid and rimfampin. In 2008, MDR was estimated to have killed 150,000.
To make matters worse, an even more strain strain has emerged, Extensively Drug-Resistant (XDR) TB. This strain is not only resistant to isoniazid and rimfampin, it is also immune to a fluoroquinolone and resistant to injectable amikacin, kanamycin or capreomycin.
With the exception of fluoroquinolones, there hasn’t been any new anti-TB antibiotics introduced for treatment.
TB was one of the first organisms that penicillin was not able to treat. The causative organism of TB, Mycobacterium tuberculosis, naturally produces a chemical called beta-lactamase which inhibits the active ingredient of penicillin, beta lactam, from working.
Recently though, researchers (http://www.einstein.yu.edu/home/news.asp?id=305 ) found that by combing two different drugs together, XDR TB could be killed in a laboratory setting. These two drugs were Clavulanic acid and meropenem ( a Carbapenem) .
Clavulanic acid by itself has no therapeutic effect. What it does do is neutralize beta-lactamases produced by bacteria. Unable to destroy the beta-lactam of the antibiotic, drugs such as penicillin can destroy the bacteria.
When Clavulanic acid is combined with a meropenem (a beta-lactam antibiotic), they can effectively destroy TB in a lab setting, but can they be used in a clinically setting?
In 2010, Belgium doctors treating a 14-year-old girl from Chechnya with XDR-TB with answered that question. With no other options to treat the acutely ill and malnourished patient, Clavulanic acid and meropenem produced positive results in eleven weeks. (http://www.stoptb.org/news/stories/2011/ns11_035.asp)
This is indeed positive news, but some cold realities have to be faced.
First of all, these drugs have to be administered intravenously. That not only requires an infrastructure to be put into place to provide patients this service, there is also the risks associated with long-term I.V. therapy (ie infection).
Secondly, there is the cost of these drugs.
Thirdly, clinical studies will have to be done to prove it’s effectiveness.
Finally, there is the possibility that resistance may develop to this combination. After all meropenem has recently been found to b useless against New Delhi metallo-beta-lactamase-1 (NDM-1) an enzyme at produces undetectable resistance to Beta-lactam drugs.
But hopefully this will prove to help with the global fight against TB.
Genotyping and Coronary Heart Disease
The quest for screening for heart disease is now gone to the genetic level. While it is still beneficial to screen using the traditional tests of cholesterol and triglyceride, the trend is to find more sensitive markers of heart disease.
One possibility was the discovery of the Trp719 Arg Polymorphism in Kinesin-Like Protein-6. Found in 30% of the population in Caucasians, African-Americans and Asians, it held promise as not only as a marker of heart disease, but by identifying patients with this variant it was found they were more responsive to Statin therapy (http://content.onlinejacc.org/cgi/content/short/51/4/449).
A new test was developed to test for this genotype to identify patients and put them on statin therapy.
Unfortunately, recent studies (http://www.medpagetoday.com/Cardiology/Dyslipidemia/25643) have indicated that testing for this genotype has no clinical value.
There is one genetic marker that has been found to be a reliable marker for cardiac disease, the Chromosome 9p21 region. In fact one study (http://www.ncbi.nlm.nih.gov/pubmed/22022235) found that the risk of MI and CVD conferred by Chromosome 9p21 SNPs appears to be modified by a prudent diet high in raw vegetables and fruits.
Could screening for this genotype be used to identify patients needing a dietary change to prevent heart disease?
If Chromosome 9p21 SNPs are a marker for heart disease that can be prevented by switching to a diet of raw fruit and vegetables, lipid testing will be just as important to make sure that the diet is reversing dyslipidaemia and heart disease risk.
On Oct.3, 2011, Warnex medical laboratories announced that they have “an exclusive distribution agreement in Canada for 10 of deCODE Genetics' DNA-based tests for assessing the risk of developing certain common diseases.” This will include the “deCODE MITM - Assesses the risk of myocardial infarction (heart attack). The test detects 8 SNPs in the sequence of the genome associated with risk of heart attack. These include the chromosome 9p21 variants discovered by deCODE, the highest impact and best validated genetic risk factors for heart attack yet found. The test provides a novel means of detecting the substantial genetic component to overall susceptibility to heart attack, risk that appears to be independent of well known risk factors such as elevated cholesterol and hypertension.”
It will be interesting if testing for chromosome 9p21 variants will supplement or replace lipd testing. As for KIF6, for now it seems that researchers agree to disagree about it’s clinical usefulness..for now.
One possibility was the discovery of the Trp719 Arg Polymorphism in Kinesin-Like Protein-6. Found in 30% of the population in Caucasians, African-Americans and Asians, it held promise as not only as a marker of heart disease, but by identifying patients with this variant it was found they were more responsive to Statin therapy (http://content.onlinejacc.org/cgi/content/short/51/4/449).
A new test was developed to test for this genotype to identify patients and put them on statin therapy.
Unfortunately, recent studies (http://www.medpagetoday.com/Cardiology/Dyslipidemia/25643) have indicated that testing for this genotype has no clinical value.
There is one genetic marker that has been found to be a reliable marker for cardiac disease, the Chromosome 9p21 region. In fact one study (http://www.ncbi.nlm.nih.gov/pubmed/22022235) found that the risk of MI and CVD conferred by Chromosome 9p21 SNPs appears to be modified by a prudent diet high in raw vegetables and fruits.
Could screening for this genotype be used to identify patients needing a dietary change to prevent heart disease?
If Chromosome 9p21 SNPs are a marker for heart disease that can be prevented by switching to a diet of raw fruit and vegetables, lipid testing will be just as important to make sure that the diet is reversing dyslipidaemia and heart disease risk.
On Oct.3, 2011, Warnex medical laboratories announced that they have “an exclusive distribution agreement in Canada for 10 of deCODE Genetics' DNA-based tests for assessing the risk of developing certain common diseases.” This will include the “deCODE MITM - Assesses the risk of myocardial infarction (heart attack). The test detects 8 SNPs in the sequence of the genome associated with risk of heart attack. These include the chromosome 9p21 variants discovered by deCODE, the highest impact and best validated genetic risk factors for heart attack yet found. The test provides a novel means of detecting the substantial genetic component to overall susceptibility to heart attack, risk that appears to be independent of well known risk factors such as elevated cholesterol and hypertension.”
It will be interesting if testing for chromosome 9p21 variants will supplement or replace lipd testing. As for KIF6, for now it seems that researchers agree to disagree about it’s clinical usefulness..for now.
Monday, December 26, 2011
Sensitivity vs. Specificity
Sensitivity vs. Specificity
When it comes to medicine, nothing is carved in stone, which is one issue this blog takes with medical dramas in the mainstream media; one test a diagnosis does not always make. In these dramas, a test is ordered, i.e. HIV, the result comes back, and the audience is lead to believe that it is infallible and the storyline continues.
Unfortunately, in the real three dimensional world of flesh and blood, this is not the case. Every diagnostic procedure has its limitations. In the medical laboratory, two terms are used to define them; specificity and sensitivity.
One can ask, aren’t these two of the same thing?
Specificity is the quality or condition of being specific and being specific is sharing or being those properties of something that allow it to be referred to a particular category.
Sensitivity is the quality of being sensitive, sensitive being capable of indicating minute differences.
(Thanks to http://www.merriam-webster.com/dictionary/specific for providing the definitions of these two terms).
So how does this relate to a diagnostic test? Every diagnostic procedure has one of two outcomes; it’s either positive or negative. The question that has to be asked though is it a true positive or a true negative? And on the other side of the coin, is it a false positive or a false negative?
This is always a little earth shattering in medicine when this simple truth is blurted out, sort of like being told that Santa Claus was actually your uncle, and that there’s no Easter Bunny. Unfortunately its reality and it goes back to the observation made in the first paragraph, one test a diagnosis does not always make.
Fortunately specificity and sensitivity help bring some stability to this shaky ground. When any diagnostic procedure is developed, it has to be tested against something referred to as the ‘Gold Standard’, the diagnostic test in use that is as close to being infallible as possible.
Thanks to examples found in other Web articles, the process of how this is done can be briefly explained.
The article is from http://en.wikipedia.org/wiki/Sensitivity_and_specificity, and the example used is a screening test for colon cancer, fecal occult blood (FOB). Now when screening for colon cancer, endoscopy is the preferred test, the ‘Gold Standard’. However, because it is more expensive and time consuming, it’s not practical or realistic to do it on every patient. Testing for hidden, or occult, blood in stool specimens is an easier screening tool for colon cancer. But how good is it compared to endoscopy?
From the example in this Wikipedia article, when FOB results are compared to endoscopy, there’s some good news and some bad news.
The good news was that as far as predicting whether a patient did not have colon cancer, FOB is quite reliable. True, it’s not 100%, but its rate of 91% can give the patient and physician confidence that there is no colon cancer present. The ability of FOB to show true negatives is high. The ability of a test to confirm true false negative results is its specificity, and FOB has good specificity.
However, the bad news is that when it comes to having true positive results, FOB’s track record isn’t as stellar. Bottom line, if the FOB is positive, a endoscopy would be highly recommended? Why? Because compared to endoscopy, FOB positive results are only truly positive 67% of the time. That means that a positive result can be wrong 33% of the time. That means on average, in every 100 patients with a positive FOB , 33 of them actually don’t have colon cancer, and will need an endoscopy to confirm it. When it comes to screening for colon cancer positives, FOB is not as sensitive as endoscopy.
Sensitivity is how well a test is at determining true positives from false positives. In this example endoscopy is the more sensitive diagnostic procedure as close as possible to but never attaining 100%.
On the other side of the coin, you want that diagnostic procedure to be specific as well. An example of this can be found at http://www.bmj.com/content/308/6943/1552.full. The disease in question being liver disease, the gold standard a liver biopsy examined by a pathologist, and the test evaluated a liver scan interpreted by a radiologist.
The good news was that a liver scan was just as reliable for diagnosing liver disease as a biopsy, catching 90% of true positives. Unfortunately, it wasn’t as good at identifying patients who didn’t have liver disease. It could only accurately report 63% of patients as true negatives. That meant that if 100 patients who had normal livers were given a liver scan, 63 of them would be correctly diagnosed of having normal livers. Good for them, but the other 37 normal patients would be misdiagnosed of having liver disease.
There’s one more observation to further muddy the waters. The specificity and sensitivity of the above tests mentioned are not absolute. One day a FOB may be more sensitive, a liver scan more specific. But no matter what no diagnostic test will have both 100% sensitivity and specificity.
When it comes to medicine, nothing is carved in stone, which is one issue this blog takes with medical dramas in the mainstream media; one test a diagnosis does not always make. In these dramas, a test is ordered, i.e. HIV, the result comes back, and the audience is lead to believe that it is infallible and the storyline continues.
Unfortunately, in the real three dimensional world of flesh and blood, this is not the case. Every diagnostic procedure has its limitations. In the medical laboratory, two terms are used to define them; specificity and sensitivity.
One can ask, aren’t these two of the same thing?
Specificity is the quality or condition of being specific and being specific is sharing or being those properties of something that allow it to be referred to a particular category.
Sensitivity is the quality of being sensitive, sensitive being capable of indicating minute differences.
(Thanks to http://www.merriam-webster.com/dictionary/specific for providing the definitions of these two terms).
So how does this relate to a diagnostic test? Every diagnostic procedure has one of two outcomes; it’s either positive or negative. The question that has to be asked though is it a true positive or a true negative? And on the other side of the coin, is it a false positive or a false negative?
This is always a little earth shattering in medicine when this simple truth is blurted out, sort of like being told that Santa Claus was actually your uncle, and that there’s no Easter Bunny. Unfortunately its reality and it goes back to the observation made in the first paragraph, one test a diagnosis does not always make.
Fortunately specificity and sensitivity help bring some stability to this shaky ground. When any diagnostic procedure is developed, it has to be tested against something referred to as the ‘Gold Standard’, the diagnostic test in use that is as close to being infallible as possible.
Thanks to examples found in other Web articles, the process of how this is done can be briefly explained.
The article is from http://en.wikipedia.org/wiki/Sensitivity_and_specificity, and the example used is a screening test for colon cancer, fecal occult blood (FOB). Now when screening for colon cancer, endoscopy is the preferred test, the ‘Gold Standard’. However, because it is more expensive and time consuming, it’s not practical or realistic to do it on every patient. Testing for hidden, or occult, blood in stool specimens is an easier screening tool for colon cancer. But how good is it compared to endoscopy?
From the example in this Wikipedia article, when FOB results are compared to endoscopy, there’s some good news and some bad news.
The good news was that as far as predicting whether a patient did not have colon cancer, FOB is quite reliable. True, it’s not 100%, but its rate of 91% can give the patient and physician confidence that there is no colon cancer present. The ability of FOB to show true negatives is high. The ability of a test to confirm true false negative results is its specificity, and FOB has good specificity.
However, the bad news is that when it comes to having true positive results, FOB’s track record isn’t as stellar. Bottom line, if the FOB is positive, a endoscopy would be highly recommended? Why? Because compared to endoscopy, FOB positive results are only truly positive 67% of the time. That means that a positive result can be wrong 33% of the time. That means on average, in every 100 patients with a positive FOB , 33 of them actually don’t have colon cancer, and will need an endoscopy to confirm it. When it comes to screening for colon cancer positives, FOB is not as sensitive as endoscopy.
Sensitivity is how well a test is at determining true positives from false positives. In this example endoscopy is the more sensitive diagnostic procedure as close as possible to but never attaining 100%.
On the other side of the coin, you want that diagnostic procedure to be specific as well. An example of this can be found at http://www.bmj.com/content/308/6943/1552.full. The disease in question being liver disease, the gold standard a liver biopsy examined by a pathologist, and the test evaluated a liver scan interpreted by a radiologist.
The good news was that a liver scan was just as reliable for diagnosing liver disease as a biopsy, catching 90% of true positives. Unfortunately, it wasn’t as good at identifying patients who didn’t have liver disease. It could only accurately report 63% of patients as true negatives. That meant that if 100 patients who had normal livers were given a liver scan, 63 of them would be correctly diagnosed of having normal livers. Good for them, but the other 37 normal patients would be misdiagnosed of having liver disease.
There’s one more observation to further muddy the waters. The specificity and sensitivity of the above tests mentioned are not absolute. One day a FOB may be more sensitive, a liver scan more specific. But no matter what no diagnostic test will have both 100% sensitivity and specificity.
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