Some labs are better at detecting disease, while others are better at confirming that a person does not have a disease.
The accuracy of lab results can be affected by many factors, like testing conditions, testing facilities, and the test itself.
While it’s impossible for lab results to be accurate 100% of the time, you can take steps to increase the chances that the results are right.
When you get a clinical lab test, it’s natural to take your results as hard facts. But certain factors can affect the accuracy of the test — like the test manufacturer and what the test is for. Some tests are accurate nearly 100% of the time, while others are well known for being unreliable.
So, before you hang your hat on your lab results, it helps to know the chances that the results may not be accurate. We’re here to break down what goes into the concept of testing accuracy and how you can maximize your chances of getting valid results.
How often are lab results wrong?
It’s nearly impossible for a test to be right 100% of the time. And every test has a different accuracy rate. We get more into the factors that affect accuracy below.
It’s also important to know that calculating accuracy is not straightforward. “Accuracy” means how well the test can correctly identify positive and negative cases. A test with 90% accuracy is expected to be correct in its results 90% of the time. But it can get complicated. To calculate this accuracy rate, scientists have to consider how often the test gets the positive results correct as well as how often the test gets the negative results correct.
False positives vs. false negatives in lab results
You may already know that a “positive” result means that you have what the test is looking for. And a “negative” result means that you don’t have what the test is looking for. But what about “false positives” and “false negatives” in the context of lab results? This stuff can be a bit more complex than meets the eye. But it’s an important part of understanding the accuracy of your lab tests.
A “false positive” is when a lab result says you have something — or something is present — when you don’t actually have it. For example, if a pregnancy test reports that you’re pregnant but you aren’t actually pregnant, this is a false positive.
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 that you have the condition when you don’t. This is the “specificity” of a test.
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. And 10 of them will get a false-positive result.
A “false negative” is a negative result when it’s actually a positive result. Here’s an example of a false-negative result: You test negative on a rapid COVID antibody test, but you actually have COVID.
If a test is not good at sensing if you have the condition of interest, it’s more likely to tell you that you do not have the condition when you actually have the condition. This is the “sensitivity” of a test. 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. Flu tests, for example, have a sensitivity between 50% and 70%. That means up to 30% 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 does disease prevalence affect lab-result 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 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 your lab results?
Aside from disease prevalence and a test’s sensitivity and specificity, there are several other factors that can affect your test results. These include:
Testing conditions: Results can be affected by testing conditions, like if you can do the test at home or if you have to do it in a provider’s office or a testing center.
The testing facility: Some labs may use different methods or equipment, and these factors may play a role in how accurate the results are.
Storage and transportation:Incorrect specimen storage and transportation can both threaten test accuracy.
Timing: Test results can also be affected by 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.
Food and drink: What you eat and drink can affect test results because certain parts of food and drinks may be absorbed into the bloodstream. And this can affect how a test detects what it’s looking for. Your provider may ask you to not eat or drink anything for a certain amount of time before a lab test. This is often the case for blood tests.
Certain medications and health status: For example, pregnancy can affect test results. Before taking drug screenings, your provider may ask you if you take any medications that might affect the results.
This is not a full list of all the factors that can affect lab results. Even though many things can play a role, you can take steps to minimize inaccurate results.
Are there ways to make sure your lab results are more accurate?
It can be stressful to get lab results that might not be accurate. To set yourself up for the most accurate test results, consider these steps:
Follow any directions your provider gives you to prepare for the test. For example, ask your provider if you can eat or drink before the test or if this could affect your results.
If you are providing a blood sample, stay hydrated. Drinking water keeps more fluid in your veins, which can make drawing your blood easier.
If you are providing a urine or stool sample, check with your provider if you have provided a large enough sample. Sometimes too little urine or stool can make it harder for the test to provide an accurate result.
Do your best to stay calm and relaxed when the lab sample is being collected. This improves the chances that you or the technician will get a good sample.
And don’t be afraid to trust your gut. If your test comes back with a result that doesn’t seem right based on what you’re feeling, talk to your provider about it. They may be able to repeat it or run other tests that can help figure out what may be going on.
What to look for in a lab result
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 want to know with certainty that you have a disease, look for a test with higher sensitivity (fewer false positives). For example, if your chances of survival improve the sooner you identify and treat a disease you may have, then sensitivity is more important than specificity.
And if you want to know with certainty that you don’t have a disease, look for a test with higher specificity (fewer false negatives). Consider COVID in this case. If you want to be sure you don’t have COVID before going back to work, specificity is more important.
You’ll also want to make sure the test itself is credible. If you’ve ordered an at-home test kit, make sure the FDA has authorized it and that it comes from a reputable manufacturer. If you go into a testing center, you’ll want to check that they are a trusted organization.
The bottom line
Getting back lab results can be stressful, especially if they don’t seem right. And it’s nearly impossible for a test to be right 100% of the time. Some tests are better at detecting disease whereas others are better at detecting the absence of a disease.
While no test is perfect, you can take steps to maximize the accuracy of your lab results. And if you aren’t sure about the results, your provider can repeat the lab or run other tests to help you figure out what may be going on.
The author is Microbiologist Certified infection control Auditor Kidney Hospital Srinagar [email protected]