Taking temperature is not a reliable way of detecting Covid-19

Scanning the forehead to determine skin surface temperature is an unreliable indicator of having Covid-19. — AZHAR MAHFOF/The Star

Making people stand in front of a scanner to have their body temperature read can result in a large number of false negatives, allowing people with Covid-19 to pass through airports and hospitals undetected.

Leading experts in physiology have suggested that taking temperature readings of a person’s fingertip and eye instead would give a significantly better and more reliable reading, and help identify those with fever.

The study, co-led by human physiologist and expert in temperature regulation Professor Mike Tipton, was published in the journal Experimental Physiology.

Prof Tipton from the University of Portsmouth in the United Kingdom says: “If scanners are not giving an accurate reading, we run the risk of falsely excluding people from places they may want or need to go, and we also risk allowing people with the virus to spread the undetected infection they have.”

The study found four key factors:

  • Temperature alone isn’t a good indicator of disease as not all who have Covid-19 have a fever, and many who do only develop one after admission to hospital.
  • Measuring skin temperature doesn’t give an accurate estimation of deep body temperature, which is raised in a fever and is impractical to measure directly.
  • A high temperature, even one taken from deep body, does not necessarily mean a person has Covid-19.
  • Taking two temperature measurements, one of the finger and the other of the eye, is likely to be a better and more reliable indicator of a fever-induced increase in deep body temperature.

Prof Tipton says: “Using a surface temperature scanner to obtain a single surface temperature, usually the forehead, is an unreliable method to detect the fever associated with Covid-19.

“Too many factors make the measurement of a skin temperature a poor surrogate for deep body temperature – skin temperature can change independently of deep body temperature for lots of reasons.

“Even if such a single measure did reflect deep body temperature reliably, other things, such as exercise, can raise deep body temperature.

“The pandemic has had a devastating global effect on all aspects of our lives, and unfortunately, it’s unlikely to be the last pandemic we face.

“It’s critical we develop a method of gauging if an individual has a fever that’s accurate and fast.”

A change in deep body temperature is a critical factor in diagnosing disease with as little as a one degree increase indicating a potential disease.

The most common symptom of 55,924 confirmed cases of Covid-19 reported in China up to Feb 22, 2020, was fever, followed by other symptoms, including dry cough, sputum production, shortness of breath, muscle or joint pain, sore throat, headache, chills, nausea or vomiting, nasal congestion, and diarrhoea.

However, the researchers say a significant proportion (at least 11%) of those with Covid-19 do not have a fever, and that fewer than half of those admitted to hospital with suspected Covid-19 had a fever.

Although the majority of positive cases go on to develop a high temperature after being admitted to hospital, they were infectious before their temperature soared.

Prof Tipton says: “We think we can improve the identification of the presence of fever using the same kit, but looking at the difference between eye and finger temperature – it’s not perfect, but it is potentially better and more reliable.”

He adds: “During the SARS (severe acute respiratory syndrome) epidemic in 2003, there was a need for a fast and effective mass-screening method and infrared thermography became, and remains, the cornerstone measurement, despite concerns over its reliability.”

A 2005 study of 1,000 people comparing forehead temperature with three different infrared thermometers gave different temperatures, ranging from 31°C to 35.6 °C.

The same infrared thermometer alone varied by as much as 2°C.

In another study, more than 80% of the 500 people tested using infra-red thermometers, gave a false negative result.

Such differences in skin temperature could be due to a range of reasons, including whether the individual has recently exercised, has an infection, sunburn or recently drunk alcohol, how close they stand to the scanner, the air temperature, how much fat they have, and even their blood pressure.

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