HSE advisory committee reports on COVID testing in the workplace
Recently, the Health & Safety Executive (HSE) asked their Workplace Health Expert Committee (WHEC) to consider how various testing regimens can be used to prevent individuals with COVID-19 from entering the workplace, particularly when they may be asymptomatic or pre-symptomatic.
Workplace Health Expert Committee (WHEC)
The WHEC is a scientific and medical expert committee whose purpose is to consider the evidence linking workplace hazards to ill health. It is tasked by the HSE with providing independent, authoritative, impartial and timely expertise on workplace health.
The Committee provides the HSE with an informed opinion on which to base policy and its latest report on workplace testing for COVID-19 has just been released.
Testing cannot stop COVID-19
As is well known, workforce testing alone cannot eliminate the risks of covid from COVID-19, but it can play a part in reducing the risk of transmission of the disease within a workforce. It is for this reason that the government is providing free testing kits for employers.
As the report states “testing achieves nothing without accompanying actions. Testing must therefore be viewed as part of a wider risk mitigation strategy”.
Furthermore, as the report highlights, “harm may arise if testing is viewed by either the organisation or individual workers as an alternative to the fundamental controls of distancing, hygiene and ventilation”.
The risks to employers are subtle, yet real. Both false positive and false negative results can cause damage to both health and earnings (because of the need to isolate). However, they can also potentially damage faith not just in the testing programme, but also in the employer/employee relationship itself.
The HSE report says, “there is a risk of unintended harm from a testing programme, particularly if implemented without adequate planning and preparation”.
The devil is in the detail
Perhaps the most useful information contained in the report is the data showing the expected accuracy of different tests in different scenarios. The data is illuminating for employers and shows some of the notable weaknesses in any workplace testing programme.
Many professionals are aware that the usefulness of a covid test varies according to the frequency of disease in any given population. If the infection rate is high amongst a population, some tests (particularly lateral flow tests) are more useful. If the infection rate is low, the rate of ‘false positives’ is likely to increase.
A test’s “negative predictive value” (NPV) measures how likely a negative test result is to reflect a truly negative situation of infection or infectivity. Conversely, a test’s “positive predictive value” (PPV) measures the rate at which a positive test truly reflects infection or infectivity.
NPV and PPV values vary according to the community prevalence of infection; as infection levels rise so will the PPV, but the NPV will fall. The report highlights the impact of this phenomenon on the accuracy of tests.
Most positive results will be incorrect
The report shows that the PPV of an average lateral flow test in a population of 1,000 people with a community infection prevalence rate of 5% is 51%. Nearly half the positive results given are likely to be incorrect.
Alarmingly, with the same population of 1,000 and a community infection prevalence rate of 0.5%, the PPV drops markedly to 17%. Four out of five positive test results are likely to be incorrect.
When the community prevalence of infection is low, the NPV of COVID-19 tests tends to be very high. A negative result in likely to be truly negative and can be relied upon. In Liverpool, for example, in a series of INNOVA antigen tests in which the rate of community infection was around 1%, the NPV was approximately 99%.
Conversely, at low community rates of infection, the PPV of any test will be commensurately low. The result of this is that an increasing proportion of positive test results will be false-positives, indeed (as shown in the report) most of them will be.
The HSE report shows that a covid test with 40% sensitivity (like the INNOVA test when used in untrained hands), a 1% infection rate and a workforce of 50 being tested, 69% of the positive results will be false positives.
Lateral flow tests need trained hands
As is also widely recognised, the sensitivity of some tests (like the INNOVA lateral flow test used by the government) has been shown by Public Health England to vary substantially according to the skill of the individual administering it.
The quality of swab sampling in particular is likely related to varying levels of reported test sensitivity, as is the type of sample used (e.g. nasal, pharyngeal or saliva).
For example, the PHE/Porton Down study of lateral flow tests found the sensitivity to be 78% when used by trained professionals. However, this fell significantly when tests were conducted by other non-specialist users.
Similarly, the mass community testing of the public in Liverpool relied on the general population to self-swab whilst supervised by trained staff. This study found sensitivity levels of 40% for the Innova lateral flow test. Our ‘lateral flow test’ debate article has more detail.
Testing may not significantly reduce workplace transmission
The report concludes that “given the issues with the accuracy of the tests, using testing to remove people from the workplace when they receive a positive test result may not significantly reduce workplace transmission risk.”
Lastly, in possibly veiled criticism, the report concludes that the issues and risks for employers when conducting workplace COVID testing “do not currently appear to be well understood by very many in society, including policy makers and some specialists advising them”.
Our advice for employers continues to be that any workforce testing programme needs very careful consideration. There are many risks for employers, as highlighted in our recent guide to covid workplace testing.
The simple steps may be most effective
A continued emphasis on good hygiene, social distancing, PPE and rapid testing of any symptomatic cases via the NHS is likely to continue to deliver the best balance of investment and risk mitigation for most employers.
The full report is available online on the HSE website.