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…

More on Lab Tests That Are Often Misinterpreted

Does My Child Need a Flu Test?

Do you need to get a flu test if you think that you have the flu?

Your child has a fever, cough, runny nose, body aches and chills.

Should you rush them to your pediatrician for a flu test?

Diagnosing the Flu with a Flu Test

While you may want to seek medical attention, depending on your child’s age and how sick they are, believe it or not, you don’t need a flu test to get diagnosed with the flu.

“If your doctor needs to know for sure whether you have the flu, there are laboratory tests that can be done.”

CDC on Diagnosing Flu

A flu test is an option though.

Most people do not need a flu test.
Most people do not need a flu test.

Is it a good option?

A necessary option?

“Most people with flu symptoms are not tested because the test results usually do not change how you are treated.”

CDC on Diagnosing Flu

While a diagnosis of the flu can be made clinically, based on your symptoms, a flu test can be a good idea:

  • to help determine the cause of an outbreak (mostly if there aren’t already a lot of flu cases in your area)
  • if someone is at high risk for flu complications

In general though, most people do not need a flu test, especially during the active part of flu season.

What’s the problem with doing a flu test?

“In January 2017, the FDA reclassified antigen-based RIDT systems into class II. This reclassification was to help improve the overall quality of flu testing. The reclassification was prompted, in part, by recognition that the poor sensitivity of some of antigen-based RIDTs resulted in misdiagnosed cases, and, according to anecdotal reports, even death.”

FDA on CLIA-Waived Rapid Flu Test Facts

Mostly, they are neither as accurate nor as easy to interpret as most folks think, even the newer versions of these tests.

Have you ever heard someone say that they tested positive for both flu A and flu B?

When a flu tests is positive for both A and B flu strains, it invalidates the test. They may have had either flu A or flu B or neither, but they almost certainly didn’t have both.

The antigen-based rapid flu tests that most doctors and clinics use, which give results in 10 or 15 minutes, are also prone to both false positive (you don’t really have the flu, even though your test was positive), and more commonly, false negative (you actually do have the flu, even though your test was negative) results, depending if flu is active at the time.

Other flu tests are available, but are more expensive and take longer to get results, so aren’t used as often. These include “rapid” nucleic acid detection based tests that can be done in a doctor’s office, rapid nucleic acid detection based tests and rapid influenza diagnostic tests that are done in a central lab, PCR tests, and viral cultures.

So why do so many people rush to the doctor to get a flu test?

Many think that if they are positive, then they can take Tamiflu or another flu medicine and get better faster.

The problem with thinking like that is that few people actually need to take Tamiflu, as at best, it only helps you get better about a day quicker than if you didn’t take it. That’s why the recommendations for Tamiflu say to reserve it for children under two to five years of age and others who might be at high risk for flu complications.

Since most other people don’t need to take Tamiflu, they don’t necessarily need a flu test or a definitive diagnosis of the flu. Again, even if they did need Tamiflu, the diagnosis of the flu could be made clinically.

And even more importantly, a negative flu test doesn’t necessarily mean that you don’t really have the flu, especially if you have classic flu symptoms in the middle of flu season. Again, a negative flu test could be a false negative.

“RIDTs may be used to help with diagnostic and treatment decisions for patients in clinical settings, such as whether to prescribe antiviral medications. However, due to the limited sensitivities and predictive values of RIDTs , negative results of RIDTs do not exclude influenza virus infection in patients with signs and symptoms suggestive of influenza. Therefore, antiviral treatment should not be withheld from patients with suspected influenza, even if they test negative.”

CDC on Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests

Have you ever had a negative flu test and the doctor still gave you Tamiflu? Then why did they do the test?

Diagnosing the Flu Without a Flu Test

If the results of flu testing aren’t going to change how you are treated, then you probably don’t need to have the flu test done in the first place.

Plus it saves you from having a swab stuck up your nose.

But kids should have flu tests, right?

Although rapid flu tests might be a little more accurate in kids than adults, it is not by much, so you are left with the same issues.

A positive test might reassure you that it really is the flu, but your child could still have the flu if their test is negative. A diagnosis and treatment decision can be made clinically, without a flu test, remembering that most older, healthy kids don’t need to be treated with Tamiflu.

We can’t skip flu season (although we sure can try if we get vaccinated and protected), but we can try and skip flu testing season.

More on Flu Tests