Thermometers: When to go (non-)contact?

The importance of body temperature as one of vital signs is well known to medical professionals and patients. Body temperature is important from the diagnostic point of view. There are different methods of measuring it, each with its own advantages. One of the biggest recent inventions in this field is the contactless thermometer, but it is not completely interchangeable with the contact one. However, the recent technological advancement has resulted in an all-in-one thermometer solution.

In this blog you will learn:

• How do we measure body temperature?
• Body temperature states
• What are the advantages of non-contact thermometers?
• What are the advantages of contact thermometers?
• Are there diagnostic solutions that combine contact and non-contact methods of measuring temperature?

How do we measure body temperature?

The gold standard for measuring core body temperature is the pulmonary artery catheter method, in which a thermistor (a temperature sensor) is placed through the catheter. This highly invasive technique is normally used only in ICU or cardiac surgery patients for additional monitoring of hemodynamic parameters. [6]

Consequently, other measurement sites (oral, axilla, rectum, etc.) and methods are used in clinical practice. They all have their own benefits and disadvantages, which have to be carefully considered before deciding which one to use in a particular situation. [7] [8] [9] [10] [11] [12] [13]

Measuring body temperature at a GP office, at home visits or in residential care needs to be fast and convenient. However, if the patient’s condition involves severe changes in body temperature, then accuracy and the ability to perform continuous monitoring are essential.

Body temperature states

Normothermia

Normal human body temperature or normothermia varies widely and depends on age (older adults generally have lower body temperature than younger adults), time of day (lower in the morning, higher in the evening), level of consciousness, emotional state, physical activity, menstrual cycle in women, and most importantly, on the site of measurement (more about that later) [1] [2] [3] [4]. Core body temperature between 36.1 °C (97 °F) and 37.2 °C (99 °F) is generally considered normal body temperature [5].

Hypothermia

Low body temperature or hypothermia is body core temperature below 35.0 °C (95.0 °F) and can be mild, moderate, severe or profound [17]. In the latter case, the patient doesn’t exhibit vital signs and their core temperature is less than 20 °C (68.0 °F), which often leads to neurological injury (brain damage) and death [18] [19].

Mortality from severe and profound hypothermia is high and ranges between 38% and 75% [21] [22] [23]. However, since hypothermia lowers the metabolic rate, the brain can withstand much longer periods of hypoxia than normal, resulting in survivals [20].

Hypothermia can be caused by exposure to cold weather (often due to alcohol intoxication) or  medical conditions (e.g. severe cases of anorexia nervosa) [14] [15]. It can also be artificially induced for medical reasons (therapeutic hypothermia); this is done to improve neurological recovery and reduce mortality in individuals with cardiac arrest and in infants with moderate or severe hypoxic-ischemic encephalopathy). [16].

All treatment for hypothermia, from mild cases that require only passive external warming, to invasive active core rewarming in severe and profound cases, is predicated on the ability to accurately measure the body core temperature. [24]

In hypothermia, the preferred measurement sites (methods) are those that correlate well with pulmonary artery catheter measurement, e.g. oesophageal, urinary bladder, rectal, and nasopharyngeal thermometers, with oesophageal temperature measurement being the best (low invasiveness coupled with good accuracy), but only in intubated patients [25] [26]. Oral, forehead, axilla, and tympanic measurements are inappropriate for use in hypothermia (low accuracy, influenced by environmental factors, etc.). They are suitable for elevated body temperature.

Fever and hyperthermia

These two conditions are on the other end of the body temperature spectrum. They are both characterised by elevated body temperature, but greatly differ in the underlying pathophysiological mechanisms, severity, and seriousness.

Hyperthermia is generally defined as a core body temperature above 37.5–38 °C (99.5–100.4 °F) that occurs without a change in the body’s temperature set point and is a symptom of the body’s overwhelmed thermoregulatory heat loss mechanisms [27]. It can range from mild, which can be self-treated, to severe. The latter is classed as a medical emergency, requiring prompt treatment (CPR, intravenous hydration, gastric or rectal lavage with iced saline, hemodialysis) at a medical facility [28] [29]. Many patients do not receive timely or adequate treatment and ultimately die as a result. It is estimated that, between 2000 and 2019, excessive heat accounted for more than 489 thousand deaths annually [30]; the most common cause of hyperthermia is heat stroke [31]. Less common causes are adverse reactions to drugs, thyrotoxicosis, pheochromocytoma, and damage to the central nervous system [32] [33] [34] [35].

Fever (usually defined as any temperature above normal human body temperature) is one of the most common medical signs. It is caused by a very wide variety of conditions, from benign to life-threatening. Most common causes include viral (the common cold, influenza, COVID-19, etc.), bacterial (e.g. urinary tract infection), and parasitic infections (malaria), followed by non-infectious diseases and conditions ranging from deep vein thrombosis (DVT) to cancer [36] [37] [38] [39] [40] [41] [42]. In difference to hyperthermia, there is a change in the body’s temperature set point (to contribute to host defence, although it may be maladaptive when the cause is non-infectious in nature) and that fever rarely requires treatment [43] [44] [45].

Differentiating between hyperthermia and fever requires taking into account other symptoms as well as environmental and other conditions in which the patient was before examination. A variety of temperature measurement methods is available. The requirements for temperature measurement in the case of fever and hyperthermia are less stringent than in hypothermia monitoring; for this reason, contact thermometers (digital, mercury, tympanic, temporal artery thermometers, etc.) and non-contact thermometers can be used.

What are the advantages of non-contact thermometers?

Technological advancement, particularly the discovery of semiconductors (used as thermistors), have dramatically transformed how we approach body temperature measurement – with the introduction of non-contact infrared (IR) thermometers (NCITs) [46]. Their greatest advantage is the ability to measure body temperature without physical contact with the body; they are most typically ‘aimed’ at the patient’s forehead. This virtually eliminates the contamination risk and consequently the need for replaceable probes, probe covers and disinfectants, which translates into decreased costs and saved time [47] [48] [49]. Additionally, the measurement is almost instantaneous.

For the aforementioned advantages, non-contact thermometers are highly suitable for use in clinical settings with a high volume of patients and therefore the need to eliminate or greatly reduce the risk of (cross-)contamination. [50] [51] [52]

NCITs can also be an excellent screening tool in other settings, for example at a GP office or an inpatient facility, particularly in case of paediatric patients, who often find other methods uncomfortable [53].

It is useful to note that, although technologically very similar, tympanic membrane thermometers are not categorized as NCITSs. The sensor probe must be physically inserted in the patient’s ear canal to obtain an accurate reading of the tympanic membrane temperature and thus comes into direct physical contact with a body part.

What are the advantages of contact thermometers?

Contact (traditional) thermometers touch the patient’s body. They are generally more accurate than NCITs and are used when accuracy is paramount (e.g. in patients with hypothermia) or there is a suspicion that a patient has borderline fever or hyperthermia, which a NCIT fails to detect [54].

There are several distinct technologies used in contact thermometers. Mercury thermometers are being phased out in some countries and regions due to mercury’s toxicity and the fragility of the glass that contains it. The most common ones in use today are digital thermometers, which use a thermistor to measure temperature.

Contact thermometers provide more accurate and reliable temperature readings than NCITs, but have to be sterilised between patients or use disposable covers to prevent (cross-)contamination. Also, the examiner should record the measurement site when measuring temperature, as there are significant differences between results [55].

Research shows that the normal body temperature range for women for oral temperature is 33.2–38.1 °C (91.8–100.6 °F), rectal 36.8–37.1 °C (98.2–98.8 °F), and tympanic 35.7–37.5 °C (96.3–99.5 °F) [55]. The range values are somewhat different for men: oral 35.7–37.7 °C (96.3–99.9 °F), rectal 36.7–37.5 °C (98.1–99.5 °F), and tympanic 35.5–37.5 °C (95.9–99.5 °F) [55].  

The measurement site should be recorded in the patient’s medical record, next to the temperature.

Are there diagnostic solutions that combine contact and contactless temperature measurement?

The answer is yes! The MESI mTABLET Thermometer combines both a NCIT and a digital contact thermometer in one digital solution. The contact thermometer is encased in the NCIT, so both are always at hand.

The NCIT has a distance indicator for optimum measurement while the contact thermometer is used with disposable shields to reduce the risk of cross-contamination. Both thermometers are wirelessly connected to a certified medical-grade tablet with an 11’’ screen. On the tablet, the user selects which thermometer will be used, enters the measurement site (in the case of contact thermometer), and activates the measurement.

After the user confirms the measurement result, it is instantly stored into the free MESI mRECORDS cloud-based storage with digital patient files. From there, it can be accessed through any web-enabled device, along with the measurement history in graph form. The MESI mTABLET can also easily be integrated with your EHR system, so all the temperature measurements can be ordered and stored there.