key: cord-1000176-figjk263
authors: Wright, William F; Mackowiak, Philip A
title: Why Temperature Screening for COVID-19 with Non-Contact Infrared Thermometers Doesn’t Work
date: 2020-12-14
journal: Open Forum Infect Dis
DOI: 10.1093/ofid/ofaa603
sha: 73a71e9454da184b59cefd5f939a82a87931a3fa
doc_id: 1000176
cord_uid: figjk263

Coronavirus disease 2019 screening can evaluate large numbers of patients while reducing healthcare exposures and limiting further spread of the virus. Temperature screening has been a focal point of case detection during the pandemic as it is one of the earliest and most frequently reported manifestations of the illness. We describe important factors to consider of screened individuals as well as the measurement process and current outcomes. Optimal temperature-based screening involves both individual and environmental factors as well as reconsideration of the current fever threshold.

higher on at least two occasions should practice social distancing with self-quarantine for 14- days (1) . In health care settings, the CDC defines fever as a forehead temperature greater than or equal to 100.0 o F (37.8 o C) (1) . In screening persons for infections requiring quarantine in the non-health care setting, the CDC defines fever as a forehead temperature ≥100.4 o F (≥38.0 o C) (4) obtained with a non-contact infrared thermometer (NCIT) (5).

Unfortunately, temperature screening programs intended to identify SARS-CoV-2-infected persons are, at best, marginally effective, because nearly half of infected persons never develop a fever (6) .

Temperature screening for SARS-CoV-2 is also an integral component of containment efforts globally. Although on the surface the screening process appears straightforward, several basic questions arise on closer examination. How, for example, did fever happen to be defined as a temperature of ≥100. Because patients' temperatures were taken in the axilla using a mercury-in-glass thermometer (personal communication), the relevance of these observations for SARS-CoV-2 screening strategies in the U.S., which rely primarily on NCITs, is uncertain.

Because temperature varies throughout the body by anatomic site, the term -body temperature‖ is meaningless. There is an axillary temperature, an oral temperature, a rectal temperature and so on, all of which differ one from another. In general, axillary temperatures are slightly lower than simultaneously obtained oral temperatures, which are lower than rectal temperatures. In the face of such variability, there is no -body temperature,‖ only the temperatures of individual body parts. -Core temperature‖generally defined as the temperature of blood in the pulmonary vein -is as close as one can get to a -body A c c e p t e d M a n u s c r i p t 6 temperature,‖ in that it is the temperature of the internal environment of the body, and influenced less by the environmental temperatures than surface temperatures such as those of the axilla, mouth or skin.. However, measuring the core temperature requires catheterization of the pulmonary artery, which is neither safe nor practical as a screening test (9) . Instead, surrogate temperatures obtained at various sites (e.g., mouth, rectum, axilla), which correlate roughly with the core temperature, are monitored clinically. Various types of thermometers have been used for this purpose, including mercury-in-glass, alcohol-in-glass, digital and Readings obtained with NCITs, which measure surface temperature (generally of the mid-forehead), are influenced by numerous human, environmental and equipment variables, all of which can affect their accuracy, reproducibility and relationship with core temperature.

These include the subject's age and gender and medications (especially antipyretic drugs) A c c e p t e d M a n u s c r i p t 7 being taken (7) . Women have slightly higher temperatures than men, African-Americans slightly higher temperatures than Caucasians (12) . In addition, temperature varies in a circadian fashion, with early morning (oral) temperatures lower on average by 1.0 o F (0.56 o C) than evening temperatures. Then there is the emissivity (the capacity to emit heat by radiation) of the surface being examined, which is influenced by a person's complexion, the wearing of makeup and sweat. Environmental factors, such as subject-to-sensor distance and ambient temperature and humidity, also affect readings obtained with NCITs (13) .

Finally, the phases of fever itself are potentially important factors determining the results obtained with NCITs. During the ascending phase of fever, a rise in core temperature occurs because of cutaneous vasoconstriction which reduces the release of heat from the body. During devervescence, cutaneous vasodilation produces the opposite effect. Because NCITs measure heat being emitted from the skin surface, both cutaneous responses can limit their capacity to detect the presence of fever (14) .

The reliability of NCIT devices is largely unknown. We are aware of only one study comparing readings obtained with such devices and an electronic thermometer, one reported Unfortunately, because 40-45% of persons with SARS-CoV-2 infections are asymptomatic (6) , any effort to identify such persons short of testing them for the virus itself would likely fail. Because mass screening for the virus is constrained by our current capacity to do so and the cost of such a program should it become available, innovative tactics for public health surveillance, such as those involving group testing (16) , crowdsourcing of digital wearable data, geolocated fever measurements from -smart thermometers‖ (i.e., thermometers paired to mobile devices) (17) and monitoring sewage sludge for SARS-CoV-2 (6) are worth considering. These ideas, like the question of how far the cutoff temperature defining a fever can be lowered without increasing the number of false-positive cases of symptomatic SARS-CoV-2 infection to an unacceptable level, will have to be determined by carefully-designed future investigations.

This perspective article did not involve human subjects and does not contain images (i.e., x-rays, MRIs, charts, photographs, etc.) or data that would reveal the identity of patients, healthcare workers, and any other individuals.

Interim Infection Prevention and Control Recommendations for Healthcare Personnel During the Coronavirus Disease 2019 (COVID-19) Pandemic

Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus-infected pneumonia in Wuhan, China

Clinical features of 69 cases with coronavirus disease 2019 in Wuhan, China

Centers for Disease Control and Prevention. Definitions of Symptoms for Reportable Illnesses

Migration and Border Health. Considerations for health screening for COVID-19 at points of entry

Prevalence of asymptomatic SARS-CoV-2 infections. A narrative review

Wunderlich and the evolution of clinical thermometry

Ocular manifestations and clinical characteristics of 534 cases of COVID-19 in China: A cross-sectional study

Temperature measurement in critically ill orally intubated adults: a comparison of pulmonary artery core, tympanic, and oral methods

Update: Public Health Response to the Coronavirus Disease

Public Health Response to the Initiation and Spread of Pandemic COVID-19 in the United States

A critical appraisal of 37 o C (98.6 o F), the upper limit of the normal body temperature and other legacies of Carl Reinhold Wunderlich

Best practices for standardized performance testing of infrared thermographs intended for fever screening

Fever. Basic Mechanisms and Management

A brief report on the normal range of forehead temperature as determined by noncontact, handheld, infrared thermometer

Group testing for severe acute respiratory syndrome-coronavirus 2 to enable rapid scale-up of testing and real-time surveillance of incidence

A c c e p t e d M a n u s c r i p t 9