How Accurate Are My Lab Test Results? Here’s What You Need to Know

When you get a clinical lab test, it’s natural to take your results as facts. However, depending on the test, the manufacturer, and how common a disease is, some tests can be incorrect as often as 50% of the time (like in the case of mammograms for breast cancer diagnosis) or accurate nearly 100% of the time (like in the case of a urine test for chlamydia). And now, with coronavirus (COVID-19) tests making news headlines, you may be wondering about their accuracy, too.

Here, we’ll break down what goes into the concept of testing accuracy and how you can maximize your chances of getting valid test results. 

How often are lab tests wrong?

It’s nearly impossible for a test to be right 100% of the time. This is because so many factors can affect your final results. For example: 

• Where the test is conducted — for example, at home, a testing site, or a clinic
• What the test is looking for
• If the test was performed as recommended
• How well the test itself can correctly identify positive and negative cases — what we refer to as accuracy 

A test with 90% accuracy is expected to be correct in its results 90% of the time. But it can get complicated because to calculate this accuracy rate, scientists take into account how often the test makes a mistake with positive results and how often it makes a mistake with negative results. They also look at how likely it is that any given person will be positive, which depends on how common the disease is in the general population. 

Let’s dive into this more below.

False positives vs. false negatives

You may already know that a “positive” means you have what the test is looking for. And a “negative” means you don’t have what the test is looking for. But what are false positives and false negatives? 

False positives

A false positive is a positive result when it should actually be negative — for example, if you took a pregnancy test and it told you you were pregnant, but you really weren’t. 

How often a test makes this mistake has to do with how well the test can correctly identify negative cases. If it’s not good at sensing that you don’t have the condition of interest, it’s more likely to tell you you have the condition when you don’t. 

This is referred to in science as specificity. If the specificity for a test is 90%, that means out of 100 people who are truly negative, it will correctly identify 90 of them as negative. 10 will get a false positive result.

False negatives

A false negative is a negative result when it should actually be positive — for example, if you took a COVID-19 antibody test and it told you hadn’t been infected, but you really had been.

A good test can sense if you have a condition or disease. This is what’s known in science as sensitivity. If the sensitivity for a test is 90%, that means out of 100 people who are truly positive, it will correctly identify 90 of them as positive. Flu tests, for example, have a sensitivity between 50% and 70%. That means up to 50% of positive results could be for people who don’t actually have the flu. 

Typically, a test is either better at catching positives or at catching negatives. A manufacturer usually can’t increase the sensitivity of a test without decreasing its specificity, and vice versa. 

How disease prevalence can affect accuracy

How rare (or common) a disease is will also affect how accurate your lab test might be. If the disease is really common, it’s more likely that a positive test result will correspond to a person who has the disease. So that means the number of false positives for that test will be lower.

For example, a lab test for hypertension, which affects over 30% of the U.S. adult population will have fewer false positives than a lab test for diabetes, which affects 10% of the U.S. population. 

What else can affect my lab test results?

Aside from a test’s sensitivity and specificity, and disease prevalence, there are several other factors that can affect your test results. Some of these have to do with the individual being tested, and some have to do with the testing conditions themselves, like whether it can be conducted at home or needs to be done in a physician’s office or a testing center. Another factor is the testing facility itself. Incorrect specimen storage and transportation can both threaten test accuracy as well.

Test results can also be affected by exactly when the test is conducted. The lab test for the flu, for example, is most accurate when it’s done within 3 to 4 days after symptoms first appear. Outside of this window, test results tend to be less accurate. The same is true for some of the current COVID-19 tests. If you are infected but haven’t yet experienced symptoms, your test may incorrectly give you a negative result (i.e., a false negative). 

What you eat or drink before a test can also affect test results. This is because certain components of food and beverages may be absorbed into the bloodstream and impact the ability of the test to detect what it’s looking for. You may be asked to refrain from eating or drinking for a certain amount of time before a lab test. This is often the case for blood tests. 

Certain medications and health status (e.g., pregnancy) can also impact test results. Before taking drug screenings, your provider may ask you if you take any medications that might affect the results. 

This is not an exhaustive list of factors that can affect test results. For the most accurate test results, always follow any directions your provider gives you to prepare for the test. 

So what should I ultimately look for in a lab test?

As we mentioned, tests usually prioritize either sensitivity (the ability to detect positive cases) or specificity (the ability to detect negative cases), but not both. If you are more concerned about knowing for sure that you don’t have a disease, you’ll want to look for a test with high specificity, one that produces fewer false negatives. This might be beneficial for tests for COVID-19, for example. You want to be sure you don’t have COVID-19 before going back to work. If you don’t realize you actually have the disease and you don’t self-quarantine, you could spread the virus to others. 

On the other hand, if you’re more concerned about knowing for sure if youhave a disease, you want a test with higher sensitivity. For example, if the disease you’re looking for is life-threatening, and your chances of survival improve the sooner you identify the disease and treat it, then sensitivity is more important than specificity.

You’ll also want to make sure the test itself is credible. If you’ve ordered an at-home test kit, make sure it’s authorized by the FDA and comes from a reputable manufacturer. If you go into a testing center, you’ll want to check that they are a trusted organization. In both cases, poor specimen storage and incorrect collection procedures can affect your results. 

Lastly, remember to ask if repeating your test could help confirm whether your first results were correct. It may involve taking the same test again or taking a different test that’s used for the same condition or disease.