More details should be provided when giving Covid-19 test results, say specialists

Reporting the level of a patient’s viral load would facilitate the management of cases

Doctors and patients should be provided with more information when given the results of Covid-19 tests to facilitate the management of cases, new research suggests.

At present, tests are reported in a binary manner, as either (virus) “detected” or “not detected”.

However, this could be enhanced by also reporting the level of the patient’s viral load, according to a study HSE public health specialists.

Hospital Report

This could be reported as “high”, “medium” or “low”, they say, though these categories would need to be defined.


“This reporting would be more accurate and informative to aid clinical and public health decisions, particularly when considered in the context of individual clinical data,” according to the study published in Eurosurveillance journal.

Currently, virtually all Covid-19 testing is carried out using polymerase chain reaction (PCR) technology to detect minute genetic fragments of the virus.

Under the process, a swab taken from the patient is mixed with liquids to extract the genetic material. In a laboratory, multiple cycles of amplification are used until the virus is detected.

Cycle threshold

The cycle threshold (ct) value is the number of cycles needed to detect the virus. Tests with a low ct value involve high amounts of genetic virus material; conversely, high ct values, where the original sample has been amplified many times, started with a small amount of virus.

In Ireland, a variety of testing platforms (manufacturers) are used in the national testing system, and each have their own specifications.

Patients are given a “detected” or “not detected” result but are not told how many ct cycles were needed to amplify the genetic material in their sample.

The study warns using ct values to influence patient management is complex and must be done with caution.

“Including the ct value on positive results may be confusing and misleading,” the authors warn, and ct values can differ by up to 6.5 cycles between different platforms.

Disease progression

However, in a hospital, where there is access to clinical data, repeat testing of a patient can give more insight into the person’s disease progression.

No test for Covid-19 can establish with certainty whether a patient is infectious or not. Critics of PCR say results with high ct values, for example above 34, should be discounted because the amount of genetic material involved is small and the patient is unlikely to be infectious.

However, the study shows the viral loads associated with a ct value of 34 can vary greatly across different platforms.

Paul Cullen

Paul Cullen

Paul Cullen is a former heath editor of The Irish Times.