Routine blood tests are not reliable for diagnosing long COVID, study finds

Current routine blood tests are not a reliable way of diagnosing long COVID, according to a new study.

The researchers, who published their findings in the Annals of Internal Medicine journal on Monday, examined if a COVID-19 infection led to changes in routine blood biomarkers, such as platelet counts or protein in the urine, that may be predictive of long COVID.

The study is part of the National Institutes of Health's (NIH) RECOVER Initiative, which seeks to better understand, diagnose, prevent and treat the condition.

"Our challenge is to discover biomarkers that can help us quickly and accurately diagnose long COVID to ensure people struggling with this disease receive the most appropriate care as soon as possible," said Dr. David Goff, director for the division of cardiovascular sciences at the NIH's National Heart, Lung, and Blood Institute, in a statement.

"Long COVID symptoms can prevent someone from returning to work or school, and may even make everyday tasks a burden, so the ability for rapid diagnosis is key," the statement continued.

Long COVID occurs when patients still have symptoms at least four weeks after they have cleared the infection. In some cases, symptoms can be experienced for months or years.

Symptoms vary and can include fatigue, difficulty breathing, headaches, brain fog, joint and muscle pain and continued loss of taste and smell, according to the Centers for Disease Control and Prevention (CDC).

Long COVID most often occurs in people who had severe illness, but anyone can develop the condition. People who are not vaccinated against COVID-19 may be at higher risk of developing long COVID, according to the CDC.

Scientists are not sure what causes long COVID but have identified risk factors including having underlying conditions or experiencing multi-system inflammatory syndrome due to COVID. There have also been studies concerning whether long COVID patients have blood biomarkers different from those who were infected with the virus but didn't develop long COVID.

For the study, researchers looked at more than 10,000 adults enrolled in the RECOVER Adult Cohort at 83 sites across the U.S. between October 2021 and 2023. Of the group, more than 8,700 had previously been infected with COVID.

Participants completed a set of surveys, a physical examination and 25 standard laboratory blood and urine tests. The participants were then followed routinely over the next two years, taking follow-up surveys and follow-up lab tests.

Researchers detected "markedly few differences in biomarkers between those with prior infection and those without," according to a press release.

One difference the team did find is that, compared to people without prior COVID infections, those with prior COVID infections were associated with small increases in HbA1c, which measures average blood sugar levels over two to three months to screen for diabetes. However, these increases disappeared after participants with pre-existing diabetes were excluded.

There were also slightly elevated levels of uACR, which measures the amount of albumin and creatinine in urine to help identify kidney damage, in long COVID patients. However, these differences were only seen in a small group and this damage may have occurred during their initial infection, the team said.

"Future work will use RECOVER's biobank of cohort samples such as blood and spinal fluid, to develop more novel laboratory-based tests that help us better understand the pathophysiology of long COVID," Dr. Kristine Erlandson, a professor of medicine and infectious disease at the University of Colorado Anschutz Medical Campus, Aurora, said in a statement.