The importance of accurate testing for Covid-19 cannot be over-emphasised. Early diagnosis of Covid-19 and patient isolation are important for both individual patient care and disease containment. Current testing, which relies on diagnosis of Covid-19 by nasopharyngeal swab PCR assay, has limited availability and is unreliable due to variable turnaround time – with test results taking days to return in many hospitals – and a high false-negative rate.

"If a patient comes in on the first day of symptom onset, typically day five after exposure, the false-negative rate is as high as 38%," said Dr Muhammad Zaman, an infectious disease specialist affiliated with Coney Island Hospital in Brooklyn. "This means a high number of potentially infectious patients are being misinformed of their true diagnosis. However, a simple blood test could dramatically lower that number and the subsequent spread of disease."

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A rapid laboratory test, the eosinophil count, readily obtained from a routine complete blood cell count (CBC) may provide actionable clinical information to aid in the early recognition of Covid-19 in patients, as well as provide prognostic information, according to new research published in The Journal of the American Osteopathic Association.

"We found that the absence of eosinophils on presentation can aid in early diagnosis, and in general, a persistent low count correlated with a poor prognosis for the patient," said Dr Zaman. "Review of the eosinophil count can be a useful tool in deciding whether to promptly isolate someone and initiate specific therapies while waiting for confirmatory test results."

In the study, eosinopenia correlated with diagnosis of Covid-19, and its persistence correlated with high disease severity and low rates of recovery. Low eosinophil count, or eosinopenia, is defined as having <100 cells/microliter. A healthy range is typically between 100-400 cells/microliter.

THE STUDY

In their early care of patients with Covid-19, researchers noticed that many manifested low or absent eosinophil counts at the time of admission. “We embarked on a study to corroborate this impression, assess its prevalence, and compare it with a viral infection (influenza) that may have a similar presentation to help us determine whether this observation has relevance in classifying patients at the time of presentation with disease that requires hospitalisation.”

Researchers compared the eosinophil results of routine CBC from the first 50 admitted Covid-19-positive patients with the eosinophil results of 50 patients with confirmed influenza infection at the time of presentation to the emergency department at Coney Island Hospital in Brooklyn. Of the patients with Covid-19, 60% had zero eosinophils at presentation, compared to 16% of influenza patients.

An additional 28% of Covid-19 patients had zero eosinophils within 48 hours of admission, thus a total of 88% had zero eosinophils during hospitalisation. "In Covid-19, eosinopenia could help to distinguish which patients likely have Covid-19," said Dr Zaman.