What Does it Mean When My Test Results Are Inconsistent?

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Question: What Does it Mean When My Test Results Are Inconsistent?

The other day I got a phone call from a very confused woman. She told me that she didn't know whether or not she had chlamydia. As she explained, her urine test was positive, but her genital culture was negative. She and her doctors had decided to take a course of antibiotics as though she was infected. Still, she didn't understand how the two tests could disagree.

The simple explanation - no diagnostic test is perfect. False STD test results can and do happen.

Answer: It depends on which test says what. 

Most modern STD tests are very good. However, no test is going to be 100% accurate 100% of the time. The measure of how good a test is has to do with its sensitivity and specificity. These, respectively, measure how good a test is at finding people who have the disease and who don't have the disease.

The importance of sensitivity is obvious to most people. Obviously, you want the test to be able to find as many cases of the disease as possible. However, many wonder why should it matter how good a test is at detecting people who do not have a disease. The answer is simple. Without being able to accurately detect someone's negative state, the test results would be overwhelmed with false positives.

A false positive result is when a test says that a person has a disease when they do not.

Conversely, a false negative result is when a test incorrectly says a person does not have a disease. Depending on the severity of the disease, and doctors' ability to treat it, one or the other type of false result may be more of a problem. For example, imagine a non-contagious disease where treatment delay doesn't have any long-term consequences but the treatment itself is grueling.

In this case, false positives are far worse than false negatives. The disease isn't going to cause big problems if a case is missed. However, the treatment might. On the other hand, if early treatment is important for good outcomes, false negatives will cause more significant problems. Doctors don't want to miss an opportunity to treat.

How often a test gives a false positive or false negative result doesn't only depend on the sensitivity and specificity of the test. It also depends on how common the disease is. The math to prove it can be found in this piece over here. Understanding that how many people have the disease really does make a big difference in testing shows why there's no simple answer to how accurate a test result is. The fact that accuracy depends on disease prevalence is why testing companies and physicians can't just give you a simple answer as to how likely your result is to be correct. It depends not only on the test but on the population it is used in.

So what do you do if you get two different results from two different diagnostic tests? It depends on the disease. Imagine that the disease is easy enough to treat, and the treatment doesn't have any serious side effects.

Then you'll want to just go with the flow and take the drugs prescribed for you. If not, then take yet another test. Depending on the type of tests involved, it generally becomes less and less likely that you would continue to have false results with each subsequent test that you take.

This is actually the principal behind most HIV testing protocols. False negatives aren't that common on HIV tests (although they do occur). However, false positives can be more of a problem. That's why most labs do a second test for anyone who initially turns out to be HIV positive. If both tests are positive, the individual in question is almost certainly infected.

Rapid tests are an exception to this rule. That is why they are primarily available in high prevalence settings. In areas where HIV is relatively common, they're very useful. The rapid test does a relatively good job of correctly diagnosing positive individuals and not vastly over diagnosing negative individuals.That's less true in areas where HIV is rarer. 


Ismail AA. When laboratory tests can mislead even when they appear plausible.  Clin Med (Lond). 2017 Jul;17(4):329-332. doi: 10.7861/clinmedicine.17-4-329.

Walensky RP, Paltiel AD. Rapid HIV testing at home: does it solve a problem or create one? Ann Intern Med. 2006 Sep 19;145(6):459-62.