Research identifies inadequate management of PPE in COVID-19 testing centers as major source of cross-contamination

Lack of glove changes at COVID-19 testing centers in Belgium has led to major sample cross-contamination and a high rate of false-positive results, with research being presented at this year’s European Congress of Clinical Microbiology and Infectious Diseases ( ECCMID) in Lisbon, Portugal, (April 23-26) found.

The introduction of large-scale PCR testing for COVID-19 has presented a number of logistical challenges, including a shortage of properly trained personnel to perform nasopharyngeal swabs.

Research from a government-funded laboratory in Belgium has identified inadequate management of PPE in testing centers as a major source of cross-contamination.

Scientists at the Federal COVID-19 Platform, Department of Laboratory Medicine UZ Leuven, Leuven, Belgium were alerted to the problem in September 2021 when they noticed that 70% of the samples taken that day from a test center in the Flemish Brabant, Flanders, had tested positive for COVID-19. The average positivity rate at the time was around 5-10%.

90% of the positive samples had a very low viral load, suggesting they had been infected with the SARS-CoV-2 virus, rather than being true positives.

Patient results were immediately withheld and a root cause analysis (an investigation of the cause of the spike in positive samples) was performed.

After excluding laboratory contamination, we have listed today’s results in chronological order by time of sample collection.


We saw that no one had tested negative after a sample was taken from a patient with a very high viral load and contacted the testing center immediately.


This led to no change of gloves, in combination with collection of samples at a high rate by a new swab and the breaking of a swab in the tube, identified as the probable source of contamination.


The protocols at this testing center were refined overnight and all patients whose results were held back were called back for a new sample the following day. 100% of them were negative.”


Bram Slechten, Principal Investigator

To assess the scale of the problem, Slechten and his colleagues then retrospectively checked four months (June-September 2021) of PCR test results from 11 testing centers for false positives.

A series of suspected contaminated samples was defined as a minimum of three low positive samples (low viral load, 4 RNA copies/mL) after a positive sample with a high viral load (>106 RNA copies/mL).

They also visited sites to assess staff.

The analysis identified potential cross-contamination events in 73% (8/11) of testing centers. The percentage of specimens suspected of being incorrectly reported as positive varied widely by day and by center. The four-month average ranged from 0% to 3.4% per test center.

The highest number of false positives at a testing center in a single day was 77 (out of 382 tests) – 20% of people tested that day. (All of these patients were given the option of retesting.)

On-site visits identified the lack of glove changes between patients as the source of cross-contamination.

“If the staff did not change gloves between each patient, it was almost certain that there would be contamination,” explains Slechten. “We identified four reasons why the glove change did not take place: it was simply not in the protocol; the correct protocol was in place but it was not followed due to lack of training new staff; not having the right size glove available; work pressure, some swabs had to take a patient every two minutes.”

More stringent PPE policies were put in place at all 11 testing centers from the end of October 2021, in response to the study results.

This included the managers responsible for informing each staff member who took patients of the importance of changing gloves and the testing centers contacted in the event of a spike in their false positive rate.

Tracking a test center revealed the impact. Before the procedure, he had a daily positivity rate of 11% and an average false positivity rate of 3.4%. But occasionally, the false positive rate reached 20%. After the intervention, the false positive rate dropped to almost zero.

The UZ-Leuven team continues to monitor false positive rates, in order to detect any isolated cross-contamination events.

In addition, Sciensano (the Belgian scientific institute for public health) alerted all Belgian laboratories to the problem in October 2021.

Researchers say most cases of cross-contamination were detected in time to withhold results and call patients back, meaning erroneous results were not reported. Some cases, however, went undetected, which meant that on certain days many patients received a poor result.

Mr. Slechten says: “Nasopharyngeal sampling involves close contact between the healthcare professional’s hand, the patient and the collection tube. It is therefore essential to change gloves between each patient.

“In high-throughput sampling, insufficiently trained staff had to sample a large number of patients in a limited time. This situation resulted in a high level of cross-contamination that had gone largely unnoticed, leading to false positives and self-infection of people self-isolating and missing work unnecessarily.

“Additionally, each false positive generates high-risk contacts who may also need to be tested, increasing the burden on labs, testing centers and contact tracing.”

He believes the false positives have artificially inflated the number of COVID-19 cases for Belgium. He says: “However, it is difficult to quantify, as we found many differences between the test centers we studied. Additionally, we only looked at test centers in part of Belgium, which makes it difficult to get the whole picture.

“It is very likely that this has also happened in other countries.

“While I do not have detailed knowledge of protocols at test centers in other countries, the focus is usually on potential events in the lab environment. However, our research provides a perfect example of the importance of looking beyond the lab and keeping an eye on the entire test chain.”

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