TESTS: "FALSE POSITIVE" [false alarm] AND "FALSE NEGATIVE" TEST
to this file/page, you have been directed to & are now on Dr. Shaw's personal
The practice of medicine is (1) an art based on (2) observation, (3) questioning and listening, and (3) scientific information. Within (3) is an attempt to "bet" on the future through the use of statistics. The determination of normal ranges is an approximation and not an absolute certainty. The reliability of the statisical gambling is strongest when the prevalence of the diagnosis being considered (the "concern" of the moment) is high within the population being checked (one such population might be females coming to a hospital emergency department with belly pain). Unfortunately, the USA medical system has (1) been conned by researchers and academicians into a scientific mindset (depending on "science") that is not justified by common sense. This state of mind has been generated by government (1) research incentives and
(2) regulations along with the intense culture within the medical field to self-protect (CYA) against medical malpractice threats.
When medical people tell you that you have had a
false-positive test or a false-negative test, it does not automatically mean that a
mistake was made. In fact, such a statement actually does not always truly even mean that the
result was even false. The result might be a simple aberration of the way that "normal ranges" are set. And the significance of the situation varies upon what kind of
- a "screening
test situation" (a test in search of a disease in a person who does not appear to have
it...example: PSA test for prostate cancer),
"diagnostic test situation" (a test for a specific, particular disease...example = lung
cancer...in a person who has a more general finding such as "a spot on the lung"...typical of
the specific disease we are concerned about),
- a "treatment
decision test situation": patient had a stroke which seems to be because a clot traveled to the
brain. Did it come through a patent foramen ovale (PFO)?
- or a
"monitoring test situation" (a test which helps doctors "keep track" of how you are
doing with a known disease...example: hemoglobin A1c in a
test is (1) a test result (such as blood [serum] PSA) or (2) finding which suggests the presence
of a disease which turns out to apparently not be there. But, another disorder may be
found that explains the result. Example:
We have a recent case example of a close friend
with a 22 gram prostate gland coming to our attention due to PSA going from 1.3 in 2003 to
5.9 in 2007 for an alarming PSA velocity of 1.16 ng/mL/year and doubling time of 1.82 years
and density quite elevated at 0.268 ng/mL/cc of gland. Twelve patterned biopsies found active
periglandular lymphocytic chronic prostatitis. This was "positive" for a diagnosis explaining
the elevated PSA parameters but "negative" for prostate cancer. So, it was "false positive"
for cancer because the test was being used as a cancer screening test. [By
2011, as with many men his age, he did finally develop prostate cancer.]
Another example would be a positive test for "flu" when tested in a non-flu time of the year or on the very early or very late margins of the flu season. A "positive" such test at those times can only be a presumptively positive (rather than definitively positive) until further details prove it. If subsequent developements prove it to be "flu", then the test result becomes definitively positive. If it was a false positive, it is incorrect to say that the original test result was "wrong". It was simply a positive test that did not pan out to be a definitely positive test. Imagine a medical practitioner trying to explain such a thing to an upset family who begins to wonder if anyone really knows what is going on!
A false-negative test is a result
or finding which suggests that the dreaded disease is not there but which, on further
investigation, such disease is/was, indeed, found to be present. False positive and false
negative results either cause unwarranted concern or unwarranted relief, and they can lead to
additional expense...as do true positive & true negative tests. Testing of any type
almost always leads to more expense3!
A borderline test is one with a
result but with the result maybe not clearly answering our question. It might be "not
negative" yet the "positive" finding does not fulfill the criteria needed to define a
positive test at a level high enough (positive enough) to trigger the beginning of a
treatment (see "false positive example above).
A 62 year old female smoker presents with headache
to the ER. CT scan of head shows lesions in the distribution of the right middle cerebral
artery (MCA). CT angiogram indicates might be atherosclerosis, arteritis, or recanalizing
thromboembolic clot. Trans-esophageal echocardiogram (TEE) is negative for atrial lesions.
There is negativity for Doppler flow through any PFO but occasional positivity for bubbles
right to left in 1 in 5 Valsalva coughs. The TEE is not negative; but the "positivity"
(bubbles) is not straightforwardly positive as an indicator for PFO
Prevalence & Test
Lab test results interpretations are based on statistics. Lab
testing for the influenza virus early in the "flu season" produces poor test-reult reliability statistics
because the actual disease is rare in patients tested at that time. The prevalence of flu is very
low at that time. So, the statistical odds are also low that a positive test is a true,
correct, "positive" test and more likely that it is a false positive...a false
Therefore, the impact of test performance
statistics weighs heavily upon how widespread the disease tested for is in that patient's
population (what percentage of that population truly has that disease at that time)...the prevalence of that disease...the probability that the disease
actually exists in that person. Test performance dramatically improves when the sought-for
disease has a high percentage chance of actually existing in members of the case population. That is, a
test for lung cancer on ALL persons will poorly perform as compared to a test on a person who
is a (1) male (2) long-time cigarette smoker (3) who has a spot (lesion) on the lung (4) which does
not have any visible calcification and (5) has a stellate shape by imaging
studies. Each of those 5 factors cummulates to increase the prevelance of the sought after disease in the population tested.
Furthermore, in the case of a screening test, such
test is hoped to have the actual effect of attempting to re-position the "positive patients" into a new
population in which the sought-for disease is more prevalent. At that moment, a positive "might" have the disease. Then, more definitive testing
and questioning is done in THAT group so as to corroborate or refute what is becoming a presumptive diagnosis.
Don't Throw Baby Out with the Bath
Some tests have awful rates as to being falsely
positive for a disease; for those, a positive result is likely reported by our lab as
"indeterminate". Yet the same test may have a great (extremely low) false negative rate, such
that "negativity" truly means negative and rules out the presence of the disease in
that patient. The very rapid SUDS test for HIV was hugely reliable when negative (the bloody-covered
emergency patient did NOT have HIV) but poorly reliable when positive (it went off the market
about 2005, unfortunately). So, positive results were reported by our lab as "indeterminate";
and the patient was tested with another test which took 24 additional hours to get a confident
result as to "positivity".
Sadly, there can be economic gain by shrewdly
playing these statistics. A medical test for an infection which is falsely positive will
likely lead to the buying of medicine to treat the infection (which really does not exist).
The patient and the prescribing doctor will never know the difference...the pharmaceutical
company which markets both (1) the test and (2) the drug to treat the infection gains (incidentally
or deliberately) by a test which has a significant false positive rate. We, the people, demand all of this "testing" in our zeal to "know RIGHT NOW!"
Some researchers ignorantly or shrewdly &
intentionally design "test-performance studies" of rates of false positivity and false
negativity in order to get a grant ($)...and without being SURE that the factors are clear as
to applicability to real-word practice situations or not. The HPV test controversy in 2005 is
But, as with the following examples... THAT'S
Some more examples
of the concept:
- In the
medical arena: a mammogram shows suspicious findings (a "positive" breast cancer
screening test), but biopsy results are negative (pathology studies were concordant and showed
a known benign breast condition that explained the mammogram findings)...a "false-positive"
situation...you were scared to death but are now relieved. This is an example of a false
positive cancer screening test. The test was truly & accurately positive in screening...it
found an abnormality. But it was false in the sense that the abnormality was not
- In the
automobile repair arena: your car won't start and is towed to a garage. The mechanic
tests the battery and says it is weak (a "positive" battery-status diagnostic test) and
replaces the battery. 24 hours later you are stranded, and further work shows that the
battery-grounding cable had actually been loose and was the real problem...an example of a
false-positive diagnostic test. The test was not wrong (it was not false and should not have
actually been negative) at all; the interpretation of the result was
- In the arena
of spousal relations: a wife notices lipstick on a husband's shirt collar (a
"positive" test) and worries about marital infidelity. But she later verifies that her husband
actually encountered his Aunt Jane at a lunch break at a local fast food outlet. Old Aunt Jane
partly missed her husband's neck with her lipstick-kiss and hit his collar...a case of a
false-positive finding (which was positive for female kiss but negative for infidelity...wife
"walked the line" of possibilities of the significance [her interpretation] of the finding
and got to the truth of it).
- In the legal
arena: a lawyer accepts the complaint of a client and ...after brief
investigation...agrees to file a lawsuit in behalf of the client and names 16 suspect people in
the suit (suggesting that the lawyer has "positive" reasons for naming them...a positive
screening/diagnostic test); 12 months later, 15 people are dropped from the lawsuit because it
became abundantly clear that those 15 were not at all liable...possibly only "named" so that
they could be forced to endure questions under oath (depositions) and worry for a year or so as
the plaintiff lawyer "fishes" for helpful evidence.
- In the
political arena: an individual desires to run for office and feels certain from his
investigation (a "positive" test of judgment) that the majority of voters favor a continuation
of video poker gambling and are not on the side of his non-gambling opponent. But, he is
out-voted by his opponent's voters. His judgment was wrong based on faulty tests of voter
sentiment...false-positive rate of sentiment indicators.
- In the
marketing arena: an entity (say, a hospital) has an annual incidence of 250 breast cancer patients treated in its Breast Cancer program. Let's say that about 245 of those patients and their families are anywhere from neutral to highly positive about their experience with the program. Most live and return to happiness in the community, building up a sizeable prevelance of program positivity in the community. But, what about those five per year who are tremendously displeased with (or focus negative blame on) the program? Depending on how broad their connections are within the community, this low-prevelance but highly toxic negative cluster can cause serious damage by swaying new cases to go to some other program. That minority prevelance can be like "one bad apple can spoil the bunch".
- Welch, H. Gilbert,
M. D., MPH, Should I Be Tested for Cancer?, 2004.
- Galen RS, Gambino
SR., Beyond Normality: The Predictive Value and Efficiency of Medical Diagnosis. New
York, NY: John Wiley and Sons; 1975.
- The Vassar Stats on-line calculator and info website, HERE.
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(Posted 21 July 1998; latest addition 2 October