Lady Gaga announced that she recently tested borderline positive for lupus. Dr. Pullen talks about lupus tests, false positives, and diagnostic uncertainty.
This article originally appeared online at DrPullen.com, part of the HCPLive network.
Lady Gaga and lupus. Is this just another publicity stunt? After all Lady Gaga is an all-star at self-promotion. In a Larry King live full-hour interview, Lady Gaga stated that she tested borderline positive for lupus. To the average listener this may sound a bit goofy. After all, lupus is a serious rheumatologic disease that is often associated with arthritis, skin rashes, kidney problems, extreme fatigue and sometimes neurologic problems. What is a borderline positive test for lupus anyway? In fact this is a common scenario for several reasons. First is that lupus is not a simple diagnosis to make. Secondly, the commonly done screening tests for systemic lupus erythematosus (lupus) is a very non-specific test. The anti-nuclear antibody (ANA) test is often ordered as part of a screening evaluation for rheumatologic symptoms. In a general medical setting 35% of patients may have an ANA test positive at a titer of 1:40. The false positive rate at a higher titer of 1:160 closer to 5%. I have no idea what Lady Gaga’s titer was, but calling the test borderline implies a low titer. In any test the chances of a true positive test as opposed to a false positive test is dependent on the chances that the individual has the condition being tested. If given the overall collection of symptoms, family history and physical findings a patient is felt to have a 10% chance of having lupus, and has a titer of 1:40 then the test can be a true positive or a false positive test. The odds of each of these are:
False Positive = .9 x .35 = 31.5% -- prevalence of non-disease x false positive rateTrue Positive = .1 x .65 = 6.5% -- prevalence of disease x true positive rateRatio of false positive tests to true positive tests = 5.1 : 1
If a patient has a vague rheumatologic set of symptoms that are unlikely to be lupus, then the chances of a false positive test may be much higher than the chances of a true positive test as shown in the hypothetical situation above.
If the titer was >1:160 then the false positive rate drops to 5%, but still in a patient where the clinical presentation suggests a 10% chance of disease the test is far from conclusive:
False positive = .9 x .05 = 4.5% -- prevalence of non-disease x false positive rateTrue positive = .1 x .95 = 9.5% -- prevalence of disease x true positive rateRatio of true positive tests to false positive tests = 2.1 : 1
There are additional rheumatologic tests, specifically anti-smooth muscle DNA and others that are more specific for lupus, and can help define these scenarios, but often a patient is left concluding that their test for lupus was “borderline.”
The other issue with lupus is that the diagnosis is not made solely based on a blood test. To make a diagnosis of lupus a patient must have at least 4 of a list of 11 diagnostic criteria, only one of which is a positive ANA titer, and only one other is another of the more specific blood tests for lupus. If a patient does not have physical findings suggestive of lupus the diagnosis cannot be conclusively made.
So Lady Gaga may or may not have lupus, but she appears to join many others in feeling that she has one or more blood tests that suggest possible lupus, but lack other lab tests or physical findings to make a diagnosis of lupus. She is certainly not a common person, but she is in a common predicament.
Ed Pullen, MD, is a board-certified family physician practicing in Puyallup, WA. He blogs at DrPullen.com — A Medical Bog for the Informed Patient.