How Can I Have False Positive or False Negative STD Test Results?

Understanding Inconsistent STD Test Results

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Portrait of a confused doctor. FS-Stock / Getty Images

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.

How STD Tests Are Designed

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. Sensitivity is how good a test is at finding people who have a disease. In contrast, specificity is how good a test is at figuring out who does NOT have the disease. Both of these factors are important in determining how well an STD test works in any given circumstances.

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.

False Positive vs. False Negative STD Tests

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 says a person does not have a disease when they actually are infected. Because STD tests aren't perfect, people who design them often have to choose whether it's better to have more false positive or more false negative tests.

Which is better depends on the severity of the disease, and doctors' ability to treat it.

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 be expensive or make people feel very sick. Therefore, it's better to under-treat than over-treat.  On the other hand, if early treatment is important for good outcomes, the decision is different.  False negatives will cause more significant problems. Doctors don't want to miss an opportunity to treat a condition early. That's true even if they accidentally treat some people who don't have the disease. 

What Cases a False Positive STD Test Result?

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. You can think about it like this. Imagine you have a population where a disease is very rare. It affects only one in every thousand people.

If a test is very good at finding the disease, it will always find that person. However, if it's not perfect at detecting people who don't have the disease, then several people will test positive who don't have it. Since there's only one truly infected person, there are more false positives than true positives! On the other hand, if half of the people have the disease, the situation is different. The disease will be detected in all of them. It will also be detected in a small number of people who aren't infected. But in that circumstance, the number of false positives is much smaller than the number of true positives.

 

In other words, how many people have the disease makes a huge difference in how tests work. That's why there is 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.

Coping with Inconsistent STD Test Results

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. (They do occur. This most often happens when someone is newly infected.)  However, false positives can be a real problem. An HIV diagnosis is so scary and stigmatizing that you don't want to tell someone they have the disease when they do not. 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 about HIV testing protocols. Their results are based on a single test. 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. 

A Word from Verywell

If you end up with inconsistent STD test results, stop and take a breath. Then talk to your doctor about what makes the most sense in how you want to proceed. If treatment is simple, you may just want to get treated for the STD -- even if you're not sure you have it. If treatment is more complex, more testing may be needed.

One of the hardest things about dealing with inconsistent STD test results is knowing how to talk to a sexual partner. My recommendation is to be open and honest. Tell them that you had results suggesting you might have an STD, but the results weren't clear. If you decided on treatment, let them know. Then, suggest they might want to get tested themselves. It's the best way for them to know how to move forward with the information you provided them. Furthermore, if they're positive, it might even help you understand your own results a little more clearly. 

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