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Routine Laboratory Blood Tests Are Not Good Enough at Diagnosing COVID-19

Man receiving nasal swab COVID-19 test.

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Key takeaways

  • A team of researchers has concluded that most blood tests are not sufficiently accurate to diagnose COVID-19.
  • A better diagnostic method is an RT-PCR test, which requires laboratory analysis of throat swabs.

A team of international researchers has concluded that most routine laboratory blood tests are not sufficiently accurate to diagnose COVID-19.

Led by Mariska M.G. Leeflang, PhD, associate professor of clinical epidemiology at Amsterdam University Medical Centers, the researchers evaluated the accuracy of 67 routine laboratory tests at diagnosing COVID-19. The November study was published in the Cochrane Database of Systematic Reviews. 

One of the best COVID-19 diagnostic tests currently available is the reverse transcriptase polymerase polymerase chain reaction (RT-PCR) test, which is also one of the most widely used methods. It usually consists of a nasopharyngeal (upper throat, through the nose) swab to collect respiratory specimen. But since RT-PCR tests are time-and labor-intensive to run, clinicians often turn to more standard blood tests to look for biomarkers of infection—a change in white blood cell count, a spike in levels of certain proteins in the blood, among others—to diagnose COVID-19. The problem with this methodology is that such biomarkers are characteristic of other diseases, too. 

“These markers, while they are great tools for helping diagnose people that have infectious processes in general, lack specificity for COVID-19," Neil W. Anderson, MD, assistant professor of pathology and immunology at Washington University School of Medicine in St. Louis, tells Verywell. He was not involved with the study. "In other words, while they may or may not be signals that a patient has disease, we see them in a lot of other disease processes."

Anderson uses a white blood cell count as an example. "We can see [white blood cell count] elevated in patients with sepsis, bacterial sepsis, other viral infections," he says. "Even non-infectious causes of illness can have elevated white blood cell counts. So they're really not specific enough to label someone as COVID-19-infected.”

What This Means For You

If you suspect that you or someone you know has COVID-19, you have a better chance at an accurate diagnosis with an RT-PCR test rather than with a routine blood test. You can find your local health department's website here to learn more about available COVID-19 testing in your area.

Evaluating Clinical Accuracy

Clinical accuracy is measured in terms of sensitivity and specificity. Sensitivity refers to the number of people with a given disease the test correctly identifies and specificity refers to the number of people without the disease the test correctly identifies. With these two metrics in mind, the researchers set out to assess the ability of routine laboratory blood tests and their corresponding biomarkers to diagnose COVID-19 by analyzing the results of 21 studies on 67 tests.

They classified the tests based on whether they evaluated:

  • Inflammation
  • Blood count
  • Liver function
  • Creatine kinase
  • Serum creatinine
  • Lactate dehydrogenase

The studies themselves were conducted in 14,126 COVID‐19 patients and 56,585 non‐COVID‐19 patients in China, Iran, Taiwan, Italy, and the U.S. In almost every patient, the RT-PCR test served as the gold standard of diagnostic accuracy. 

“PCR is a really, really great technology, and it's really allowed us to make the specific diagnosis of a lot of infectious diseases," Anderson says. "And the reason why it's so specific is because when you're doing PCR, you're measuring the specific genetic signature, the nucleic acid sequence of the pathogen of interest."

In their analysis, the researchers found that only three of the 67 tests had both sensitivity and specificity above 50%—far below the 80% baseline required by medical standards. Two of the tests measured an increase in blood levels of proteins involved in regulating inflammatory immune responses, interleukin-6 (IL-6) and C-reactive protein (CRP), while the third measured a decrease in lymphocyte count, a type of white blood cell produced in the bone marrow.

The majority of the tests, however, proved even less fit for clinical use. One test measuring an increase in monocyte count had a specificity of 13%. Another measuring serum creatinine had a specificity of 7%. And one test measuring an increase in procalcitonin had a sensitivity of only 3%. 

Diagnostic Tests Have Low Accuracy

Given the low overall level of accuracy, the researchers determined that none of the blood tests studied “performed well enough to be a standalone diagnostic test for COVID‐19 nor to prioritize patients for treatment.” 

“Basically, short of direct detection of the organism, there are no clearly predictive biomarkers [of COVID-19],” Yuka Manabe, MD, associate director of global health research and Innovation at the Johns Hopkins University School of Medicine, tells Verywell. 

This conclusion, though negative, is potentially important for public health management of the pandemic, Anderson says. 

“Even though it's disappointing that we can't rely on any commonly-available basic laboratory tests to make the diagnosis of COVID-19, it's important we know that so that we don't, so that we don't try doing it as a way to address shortages of the proper testing," he says. "If nothing else, a study like this shows us how important it is to have adequate amounts of proper testing available to our hospitals."

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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  1. Stegeman I, Ochodo E, Guleid F et al. Routine laboratory testing to determine if a patient has COVID-19. Cochrane Database of Systematic Reviews. 2020. doi:10.1002/14651858.cd013787