TESTS: "FALSE POSITIVE" AND "FALSE NEGATIVE" TEST RESULTS

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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 false. And the significance of the situation varies upon whether it was: what kind of test?

  1.  a "screening test" (a test in search of a disease in a person who does not appear to have it...example: PSA test for prostate cancer), 
  2. a "diagnostic test" (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), 
  3. a "treatment decision test": patient had a stroke which seems to be because a clot traveled to the brain. Did it come through a patent foramen ovale (PFO)?
  4. or a "monitoring test" (a test which helps doctors "keep track" of how you are doing with a known disease...example: hemoglobin A1c in a diabetic). 

A false-positive test is a test result (such as PSA) or 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 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.

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 the result might not clearly answer our question. It might be "not negative" yet the "positive" finding does not fulfill the criteria needed to define a positive test which is 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 patching.

Prevalence & Test Performance:

The impact of test performance statistics weighs heavily upon how common the disease tested for is in that patient's population...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 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 smokers (3) who has a spot on the lung (4) which does not have any visible calcification and (5) has a stellate shape by imaging studies.

Furthermore, in the case of a screening test, such test has the actual effect of attempting to re-position the patient into a population in which the sought-for disease is more prevalent. Then, more definitive testing is done in THAT group.

Don't Throw Baby Out with the Bath Water:

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".

Ignorance & Abuse:

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 the test and the drug to treat the infection gains (incidentally or deliberately) by a test which has a significant false positive rate.

Some researchers ignorantly or shrewdly & intentionally design "performance test 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 an example1.

But, as with the following examples... THAT'S LIFE!!!


Some more examples of the concept:

  1. 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 cancer
  2. 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 inaccurate
  3. 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...the interpretation...of the finding and got to the truth of it).
  4. 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.
  5. 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.

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References:

  1. Making a Valid Point About HPV Tests, CAP Today, September 2005. [CAP website, CAP Today page]
  2. Welch, H. Gilbert, M. D., MPH, Should I Be Tested for Cancer?, 2004.
  3. Galen RS, Gambino SR., Beyond Normality: The Predictive Value and Efficiency of Medical Diagnosis. New York, NY: John Wiley and Sons; 1975.

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(Posted 21 July 1998; latest modification 26 June 2008)

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