Follow-up Covid Tests Proving Problematic

Dr Graham Fry, Tropical Medicine, Trinity College Dublin, Tropical Medical Bureau (Ireland) and Nomad Travel (UK), is calling on the Irish and UK governments to introduce new PCR certification guidelines for return to work and travel protocols.

PCR Ct rates should be evaluated by a medical practitioner, where a follow-up PCR test takes place two weeks after the initial positive result. (A high Cycle threshold (Ct) indicates that the analyser had to ‘multiply’ the sample many times [1]).

For the purposes of post Covid testing the simple binary ‘detected’ / ‘not-detected’ reporting system needs to be updated to reflect the differences in Ct rates.

Binary reporting does not separate out those who are currently infected (low Ct, e.g. 18) from those who had the virus weeks or months ago (high Ct, e.g. 35), where viral fragments remain and where the person has no symptoms and is not infectious.

VIDEO: MD, Michael Collins, interviews Dr Graham Fry, Founder and Group Medical Director

Increased Numbers Presenting for Post Covid Tests.

Over the month of January in Ireland and the UK we have seen a very considerable increase in the numbers of our population diagnosed with Covid19. In the majority of cases the RT-PCR method has been used to identify the genetic material of the Covid virus in the patient’s oral and nasal cavities. The results of these tests are almost always shown as simply a ‘detected’ or ‘not detected’ report.

More accurate PCR results are possible, but government guidelines are too vague.

PCR has been used for decades and is still internationally regarded as the ‘gold standard’ for identifying various viral infections.

A positive, detected result is reported when any level of the genetic material is identified during the PCR process. This technique relies on an amplification process where the sample undergoes regular cycles to identify if any genetic viral material is present.

The current government guidelines on how to use and interpret Ct scores are too vague, especially on issues relating to certification for travel or return to work. Ct rates are important and should be a determinant in conjunction with a clinical assessment, especially for follow-up post infection tests. Frequently labs do not list Ct rates when reporting Covid test results.

HSE guidance PDF ( See also notes below.

Return to Work and Travel

Key workers and front-line staff are increasingly unable to return to work or travel as a result of these inadequate and ‘false positive’ PCR results.

The HSE in Ireland and NHS in the UK have thousands of key front-line staff out of work because tiny or old viral fragments are being detected at high Ct levels when they are tested at 10 or 14 days post initial infection. Keyworkers should be allowed return to work if their initial positive PCR is more than two weeks old, less than 12 weeks and if their second result has a high Ct rate (e.g. 35) and where a medical practitioner is involved in the certification.

In fact, those who had the virus weeks back, with a high Ct rate on a follow-up test, are likely to have some level of protection against the virus, yet they are prevented from re-entering the workforce because the Ct rate is not being taken into account.

This will become a much larger problem in the coming weeks as more people are diagnosed and even more so when travel and business open-up again fully.

For example, on one day, last Saturday (30 Jan 2021), TMB had three such results. Dr Fry explains, ‘One of the patients was due to fly back to his work in the Netherlands and another was trying to get to Spain on an essential business trip. Another was trying to fly back to his work in UAE but was a confirmed Covid case from 3 weeks ago’.

All the above cases had high Ct scores on their follow-up PCR tests but their certificates only listed their result as ‘detected’ which meant they were unable to travel, even though they had no symptoms and were no longer infectious.

Covid ‘detected, not-detected’ no longer an accurate or appropriate indicator by itself.

[1] Fundamentally, a high Cycle threshold (Ct) indicates that the analyser had to ‘multiply’ the sample many times before the genetic material was ‘detected’. This means that the higher the Ct result, the lower the amount of viral genetic material which is present. Higher Ct levels generally mean that either the swab was taken very early in the infection process or alternatively sometime after the infection has occurred and the test is just picking up residual material which can remain within the oral and nasal passages for weeks following infection.

Nevertheless, the fact that residual genetic material can be identified for weeks (and occasionally months) following a confirmed infection creates a distinct problem for key workers and those planning to travel, and also others where they are required to have a ‘non detected’ result before returning to work etc. Frequently this may mean that an individual who has had a confirmed case of Covid may unnecessarily be unable to travel or return to work while awaiting a ‘not detected’ result, which could take months, using the current binary method.

Internationally it is widely accepted that in the vast majority of cases an individual with a confirmed Covid19 infection will remain infectious to others for between 10 to 14 days and only in very exceptional circumstances will that duration be extended. After that period of time, they should be allowed out from ‘self-isolation’ and proceed to follow the normal government standards in place for the rest of the population. In fact, these individuals could be expected to have some personal protection against subsequent infections for a currently ill-defined period of time, but almost certainly for at least a 3 month period.

To overcome this problem employers and airlines/governments should be moving to accept certification from a registered medical practitioner where an individual had Covid19 identified greater than 2 weeks and less 12 weeks (with a high Ct) and so can be recognised as being non-infectious to others and allow return to work / travel etc.


HSE Guidance on the management of weak positive (high Ct value) PCR results in the setting of testing individuals for SARS-CoV-2

page 4:  Ct (cycle threshold) values represent the number of cycles of amplification elapsed before the test system signals detection of the target. In general terms, the higher the Ct value the lower the quantity of virus target present in the sample. Precise definition of what constitutes a high or very high Ct value is difficult because a Ct value is not comparable to the quantitative output from a calibrated assay. The Ct value for a given sample will be different in different laboratories depending on the test platform. In general terms for this report a Ct value of 30 or greater is considered a high Ct value and a value of 35 or greater is considered a very high Ct value. It is appropriate for laboratories to adjust these thresholds based on their experience with the platform they are using.

page 9:  11. There are very few reports of viable SARS-CoV-2 virus being retrieved in culture from clinical specimens with a Ct value of >34.


Further reading: 


Duration of Isolation and Precautions for Adults with COVID-19 | CDC



COVID-19: PHE assessment of laboratory tests and assays – GOV.UK (



For comments / interview opportunities, please contact Niamh Waters,, +353 860492394

Dr Graham Fry – Founder and Group Medical Director

The Tropical Medical Bureau was founded in 1986 by Dr Graham Fry. As the leading Tropical Medical Consultant in the country, Dr Fry plays a pivotal role in the organisation, whether consulting with patients, lecturing to medical students in Trinity College or dealing with media focussed tropical issues on radio and television. His enormous medical experience and expertise in technology have combined to ensure that the TMB occupies national pride of place in tropical medical care.

About Tropical Medical Bureau Group

The Tropical Medical Bureau Group (encompassing Nomad Travel in the UK) is the leading travel medicine clinic group in Ireland, specialists in providing the best information and medical care available for the international traveller. Collectively, the TMB Group sees approximately 60,000 patients on an annual basis, ranging from the private holiday maker to employees of commercial companies, and overseas volunteers to NGO’s and Government departments, all of whom benefit from the expertise of our highly experienced clinic staff.

Since it was founded in 1988, TMB has undergone major growth and expansion to currently encompass 30 clinics, 20 clinics in Ireland (four base clinics and 16 associate clinics). In 2016, TMB acquired Nomad Travel Clinics in the UK. Nomad Travel Clinics has 25 years’ experience in travel health, pharmacy & retail, throughout 10 clinics across the UK.

What distinguishes TMB is our vast experience and expertise, as well as our superb patient aftercare. All our patients have a detailed consultation with one of our specialised healthcare staff. This covers everything from food and water risks, mosquito avoidance, malaria and thrombosis, as well as risks associated from altitude exposure, and also Zika and COVID-19 risks, which may be associated with your trip. Following a detailed consultation, we then decide on which travel vaccines and malaria tablets are appropriate. Our specialised travel medicine software is constantly being updated with travel news, so if anything is happening in your destination that you need to be aware of, we’ll make sure you are.