Cool Eyes on Fever Screening

Thermography has been a hot topic this year, due to the need for quicker diag­nostics to detect and prevent the spread of Covid-19. Noncontact infrared thermo­meters (NCITs) are currently a primary tool for fever screening, but their widespread use has been prone to inaccuracy. A related medical technology, thermo­graphy using infrared thermo­graphs (IRTs), enables increased options for tempera­ture esti­mation with greater accuracy. Although the use of thermo­graphy as a stand-alone detection method for Covid-19 is unlikely to prevent spread, emerging evidence and international consensus suggest that it is indeed possible to use IRTs effec­tively for detecting elevated body tempera­tures.

Thermal image of FDA scientist Q. Wang. (Source: Q. Wang)

Now, Quanzeng Wang and collaborators at the Center for Devices and Radio­logical Health of the US Food and Drug Administration (FDA) evaluated the use of IRTs under stan­dardized conditions. They examined clinical data for more than 500 demo­graphically diverse indi­viduals, including 47 subjects with elevated oral tempera­tures, correlating facial tempera­tures to oral under-the-tongue thermometer measure­ments. The team confirmed the utility of inter­nationally recognized (ISO) standards for obtaining consis­tently accurate results, and specifically reported the efficacy of ISO methods, including facial area targets.

IRTs camera displays a two-dimensional tempera­ture distri­bution that correlates to tempera­ture levels at the source. A human face typically shows varying temperatures, warmer where the larger arteries are closer to the surface of the skin such as the inner canthi (the eye corners where the tear ducts reside) and temples. Like any technology, IRTs are subject to fundamental device and performance variations, which affect overall accuracy when used in medical practice. To minimize such variations, the researchers incorporated the ISO guidelines in their experiments (e.g., controlling ambient conditions, requiring individuals to acclimatize for fifteen minutes prior to screening, etc.). They reported excellent IRT performance with clear corre­lations between IRT readings and oral temperature baseline data.

Aside from demons­trating the effec­tiveness of stan­dardized methods for fever screening, some key insights emerged from the team’s analysis. One of the most signi­ficant insights was that targeting the full face for tempera­ture screening resulted in greater accuracy than narrowly targeting the inner canthi, as recommended by the ISO guide­lines. This insight could be helpful in establishing future directions with these devices, as full-face imaging is much easier to implement since it does not require complex facial feature detection software to identify the inner canthi regions. The next-best perfor­mance included the extended inner canthi area that includes but is wider than the inner canthi regions recommen­ded by the ISO guidelines. Targeting the narrow canthi regions produced third-best accuracy.

FDA researchers calcu­lated a series of cutoff levels to optimize the sensi­tivity and speci­ficity values of the IRT readings. Although there is no uniform consensus on what tempera­ture constitutes a fever, an oral temperature in the range of 37.5 °C to 38 °C is most commonly used in the medical profession as the threshold. The scientists caution that deter­mining an appropriate cutoff temperature for real-world fever screening involves complex consi­derations, such as the likely rate of false positives or negatives, as well as such matters as delay times, staffing, asymptomatic illness, and related costs to health agencies.

If conducted according to standar­dized protocols to ensure accuracy, IRT-based fever screening may effectively identify indi­viduals with elevated body tempera­tures including low-grade fevers (oral tempera­ture ≥ 37.5 °C) associated with early-stage infections and moderate symptoms. According to Zane Arp, director of the division of biomedical physics at the FDA, “IRT-based fever screening, especially in regard to addressing Covid-19, should be one element in a multi­layered diagnostic process that would include other helpful tools. When used in combination with other diag­nostics and medical screening it is a much more useful tool in detecting those who are ill.” For Covid-19 in particular, the FDA has published fact sheets on the proper use and limi­tations of these systems. (Source: SPIE)

Reference: Y. Zhou et al.: Clinical evaluation of fever-screening thermography: impact of consensus guidelines and facial measurement location, J. Biomed. Opt., 25, 097002 (2020); DOI: 10.1117/1.JBO.25.9.097002

Link: Centre for Devices and Radiological Health, U.S. Food and Drug Administration, Silver Spring, USA

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