Lab Tests That Are Often Misinterpreted

To get the most accurate results and avoid false positive and false negative results, you want to use the right test for the right patient, and then know how to interpret the results correctly.

There are a lot of good reasons that most doctors should do fewer lab tests.

For one thing, many are simply unnecessary.

And few tests are inexpensive.

Another reason, one that you likely haven’t thought of, is that sometimes lab tests are misinterpreted, leading to unnecessary treatments.

Lab Tests That Are Often Misinterpreted

In addition to false positive and false negative test results, which are an inherent risk with almost any test, you sometimes run the risk that your doctor doesn’t truly understand how to interpret the results of the test they ordered.

How is that possible?

Consider Lyme disease testing.

Unless you live in or visited an area with ticks that cause Lyme disease and you have symptoms of Lyme disease, then you don’t need to be tested for Lyme disease. If you do get tested, you doctor should use two-tiered testing – an EIA or IFA test first, and if positive, Western blot testing.

The CDC recommends two-tiered testing for Lyme disease.

How do you know if your Western Blot test is positive?

A positive IgM Western blot for Lyme disease requires at least two of the following bands of the test to be positive:

  1. 24 kDa (OspC)
  2. 39 kDa (BmpA)
  3. 41 kDa (Fla)

And a positive IgG Western blot for Lyme disease requires at least five of the following bands of the test to be positive:

  1. 18 kDa
  2. 21 kDa (OspC)
  3. 28 kDa
  4. 30 kDa
  5. 39 kDa (BmpA)
  6. 41 kDa (Fla)
  7. 45 kDa
  8. 58 kDa (not GroEL)
  9. 66 kDa
  10. 93 kDa (2)

What happens if someone only sees one of the IgM bands or four of the IgG bands? Are they going to know it is a negative test or are they going to wonder if they have Lyme disease?

Still, that doesn’t mean that you should never test patients for Lyme disease. You just want to use the right test for the right patient, and then know how to interpret the results correctly.

What other tests are often misused or can be easily misinterpreted?

  • blood allergy tests – Ever been told you’re child is allergic to everything? That’s likely because instead of a simple positive or negative result, blood allergy tests are prone to false positive results
  • the PPD test – it is important to understand that interpreting the tuberculin skin test depends on the child’s risk factors and that a previous BCG vaccine can trigger a false positive
  • rapid strep tests – prone to false positive results, picking up strep carriers, especially if you test kids who do not have classic symptoms of strep throat
  • rapid flu tests – prone to false positive results if you test when flu activity is low
  • thyroid function tests
  • monospot test – this is a non-specific test, so is not just for mono and most experts recommend that it no longer be used
  • EBV titers – titers of Epstein-Barr virus (EBV) antigens, including viral capsid antigen (VCA), Early antigen (EA), and EBV nuclear antigen (EBNA) can all appear at different points in your infection, from early on to years after you have recovered. Many persist for the rest of your life after you have had mono, which some folks confuse as a new infection or a relapse.
  • vaccine titers
  • ANA – while your anti-nuclear antibody test should typically be negative and a positive ANA can be a sign of arthritis, it is also very common for kids without any problems to have a positive or elevated ANA
  • WBC
  • vitamin D levels
  • drug testing
  • tox screening
  • covid-19 tests
  • EEGs

Why are these tests so easily misinterpreted?

False Positive Test Results

For one thing, many people underestimate the risk of false positive test results.

That’s why it is important to remember that a positive test doesn’t necessarily mean 100% that you have any specific disease or condition. It just means that you have a positive test.

“EEG will be negative in a large portion of patients with epilepsy, and may be positive in patients without epilepsy. False positive EEG findings commonly lead to unnecessary use of antiepileptic drugs and may delay the syncope diagnosis and treatment. EEGs are most helpful in specific situations when there is high pre-test probability for epilepsy based on history and exam, and clinical presentation.”

Do not routinely order electroencephalogram (EEG) as part of initial syncope work-up.

The fact that you can actually have a false positive EEG test should help you understand this whole issue a little better.

So how do you reduce the chance that you will have a false positive test result – or a false negative for that matter?

“A given test will have a higher positive predictive value in those patients with a higher prior probability of disease.”

Sensitivity, Specificity, and Predictive Values of Diagnostic and Screening Tests

You have to understand the sensitivity, specificity, and predictive values of the tests you use. And the things that influence them.

“The positive and negative predictive values vary considerably depending upon the prevalence of influenza (level of influenza activity) in the patient population being tested.”

Rapid Diagnostic Testing for Influenza: Information for Clinical Laboratory Directors

For example, when no one has the flu and disease prevalence is low, you are more likely to have false-positive rapid antigen test results. So that positive flu test this year, when no one has the flu might not actually mean that you have the flu either. It is probably a false positive, which makes you wonder why the test was done in the first place…

And know that you can’t just test everyone for everything…

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Author: Vincent Iannelli, MD

Vincent Iannelli, MD