How to Take a Rectal Temperature for Adults

Rectal temperature measurement in adults follows a straightforward procedure: a lubricated digital thermometer is gently inserted about 2.5 to 4 centimeters (1 to 1.5 inches) into the rectum while the person lies on their side, and left in place until it beeps. The method remains the clinical reference standard for core body temperature because it consistently tracks closer to true internal temperature than oral, ear, or forehead readings. Knowing how to do it properly, when it actually matters, and when to avoid it can make a real difference in managing fever, heat illness, or hypothermia at home or in a care setting.

Why Rectal Temperature Is Still the Reference Standard

Every other common thermometer site is compared against rectal temperature when researchers test accuracy, and that is not just tradition. The rectum is surrounded by a dense network of blood vessels and sits deep enough inside the body that ambient air, recent eating, or mouth breathing cannot skew the reading the way they do with oral or forehead measurements. In studies comparing rectal readings to the temperature of blood flowing through the pulmonary artery, which is about as close to true core temperature as you can get, rectal measurements ran only about 0.1°C higher on average, with a correlation of 0.99.¹ That near-perfect agreement is why sports medicine guidelines still call rectal temperature the most reliably available method for monitoring core temperature during heat illness on the field.²

Other sites consistently underread when it matters most. In emergency departments, oral thermometers detected a fever of 38°C or higher only about 37% of the time when rectal temperature confirmed the fever was present. Ear and temporal artery thermometers did better, catching around 68–71% of those fevers, but still missed roughly a third.³ In a study of adults with compromised immune systems, axillary readings performed worst, detecting fever only about 35% of the time.⁴ When you genuinely need to know whether someone has a fever versus just feeling warm, rectal temperature catches what other methods miss.

Step-by-Step Procedure

The process is simple, but small details affect both comfort and accuracy. You will need a digital thermometer (ideally one dedicated to rectal use), a water-based lubricant, disposable probe covers if available, and gloves.

Position: The person should lie on their left side with their knees drawn slightly toward the chest. This is sometimes called the Sims position. It relaxes the rectal muscles and gives you a clear angle. Lying flat on the stomach also works, though side-lying is generally more comfortable for adults.
Prepare the thermometer: Slip a disposable probe cover over the tip if you have one. Apply a generous amount of water-based lubricant to the covered tip. Do not use petroleum jelly with a probe cover, as it can degrade certain plastics.
Insert gently: Separate the buttocks with one hand. With the other, slowly guide the thermometer tip into the rectum, aiming it slightly toward the navel rather than straight in. Insert it about 2.5 to 4 centimeters (roughly 1 to 1.5 inches). You should feel minimal resistance. If you meet resistance, stop and reposition rather than pushing harder.
Hold in place: Keep holding the thermometer throughout. Never let go of it and never leave the person unattended with the thermometer inserted. Most digital thermometers beep within 15 to 60 seconds.
Remove and read: Once the thermometer signals it is done, gently withdraw it, read the display, and discard the probe cover. Clean the thermometer with rubbing alcohol or soap and water according to the manufacturer’s instructions.

A normal rectal temperature in adults typically falls between 36.6°C and 38.0°C (roughly 97.9°F to 100.4°F). A rectal reading of 38.0°C (100.4°F) or above is generally considered a fever. Because rectal readings run slightly higher than oral ones, do not “subtract a degree” and compare the number directly to oral thresholds you might be familiar with. Treat the rectal number at face value against rectal reference ranges.

When Rectal Temperature Is the Right Choice

For everyday home use in a healthy adult who can hold a thermometer under the tongue, oral temperature is usually good enough. Rectal measurement becomes the better option in specific situations where accuracy has real consequences.

Heat emergencies are the clearest example. When someone collapses from suspected heat stroke during exercise or outdoor work, the treatment decision hinges on knowing whether core temperature has crossed a dangerous threshold. Guidelines for cold-water immersion treatment of exertional heat stroke rely on continuous rectal temperature monitoring to determine when cooling should stop.⁵ An ear or forehead reading in that scenario can be dangerously unreliable because sweat, flushed skin, and environmental heat all interfere with surface-based methods.

Hypothermia is the flip side of the same coin. When someone has been exposed to cold long enough that core temperature may have dropped below 35°C (95°F), standard oral thermometers often cannot read that low, and peripheral sites like the armpit are heavily affected by the cold skin itself. Rectal temperature gives a much better picture of what is happening internally.

Other common indications include patients who cannot keep their mouth closed around a thermometer, whether due to confusion, mouth breathing on supplemental oxygen, recent oral surgery, or an altered level of consciousness. Caregivers monitoring a bedridden adult with a serious infection may also find rectal readings more consistent than trying to get reliable oral measurements from someone who is shivering or delirious.

Who Should Avoid Rectal Temperature Measurement

Rectal thermometry is not appropriate for everyone. Physical manipulation of the rectum, even with a thin thermometer tip, carries a small but real risk of perforation. A case report documented a thermometer migrating into the pelvic cavity, an outcome the authors attributed to the fragility of the rectal wall and the need for careful technique.⁶ While that degree of complication is rare, it illustrates why caution matters in people with conditions affecting the lower gastrointestinal tract.

You should avoid rectal temperature measurement in adults who have recent rectal surgery, hemorrhoids that are actively bleeding or severely inflamed, rectal prolapse, or any known injury to the anal or rectal area. People with very low platelet counts or other bleeding disorders are also at increased risk from even minor mucosal trauma.

The concern about rectal manipulation triggering dangerous heart rhythm changes through the vagus nerve has been debated for decades, particularly for patients who have just had a heart attack. Some researchers have argued that rectal stimulation can slow the heart’s natural pacemaker and potentially trigger abnormal rhythms in a vulnerable heart.⁷ In practice, many critical-care units avoid rectal temperature in acute cardiac patients as a precaution, even though the actual risk in a calm, controlled measurement is likely very low.

The Neutropenia Question

One longstanding concern involves patients whose white blood cell counts have dropped dangerously low, a condition called neutropenia that is common during chemotherapy. The traditional worry was that inserting a rectal thermometer could introduce bacteria through tiny breaks in the rectal lining, potentially causing bloodstream infections in someone whose immune system cannot fight them off. A study examining this directly found that patients who had their temperature taken rectally did not experience a higher rate of documented bloodstream infections or increased hospital mortality compared to those measured orally.⁸ Despite this reassuring data, most oncology guidelines still recommend avoiding rectal thermometry in neutropenic patients out of an abundance of caution. If you are caring for someone going through chemotherapy, follow the guidance of their medical team on this point.

The Lag Problem

One genuine limitation of rectal temperature that is worth understanding: it responds slowly to rapid changes in core temperature. If someone’s internal temperature is rising or falling quickly, the rectal reading can lag behind what is actually happening in the blood. This has been documented in research showing that when arterial blood temperature changes rapidly, rectal temperature takes time to catch up.⁹

For a single reading to check whether someone has a fever at home, this lag is irrelevant. The person’s temperature is not changing second by second. But in clinical scenarios like actively cooling a heat-stroke patient, the lag matters. If you stop cooling when the rectal thermometer hits the target temperature, the person’s actual core temperature may already be lower than the rectal reading suggests. This is why treatment protocols for exertional heat stroke use specific rectal temperature cutoffs calibrated to account for this delay, stopping immersion at a rectal reading of about 38.6°C rather than waiting for normal temperature, because they know the core will continue dropping after cooling stops.⁵ Esophageal temperature probes track changes faster, but they require a probe placed down the throat and are essentially limited to intubated hospital patients.

How Far Off Are Other Methods

If you have ever wondered whether the thermometer you swipe across a forehead or point into an ear is “close enough,” the short answer is that it depends on why you are checking. For a general wellness check on an otherwise healthy adult, oral and tympanic readings are fine. The trouble comes when the stakes are higher.

A meta-analysis of oral thermometry studies found that oral temperatures consistently underestimate fever compared to rectal readings, with the underestimation being largest when rectal temperature was used as the reference standard.¹⁰ Axillary measurement performs worst, with one study in a tropical clinical setting finding that armpit readings ran a full 0.8°C lower than rectal readings on average.¹¹ That gap matters clinically. If someone’s true core temperature is 38.5°C, an axillary thermometer might read 37.7°C, which looks normal and could delay treatment.

The pattern is consistent across studies: every peripheral measurement method tends to read lower than rectal, and the gap widens when the person actually has a fever. This is not a flaw in those thermometers so much as a consequence of where they are measuring. Skin surfaces and airways are cooler than the body’s core, and the difference grows as internal temperature rises.

Comfort, Privacy, and Practical Barriers

The biggest reason rectal temperature is not used more often, despite its accuracy, has nothing to do with science. It is uncomfortable and embarrassing. In a qualitative study of emergency prehospital personnel, placing a rectal probe was described as embarrassing and uncomfortable for both patients and providers, and was seen as an invasion of privacy, particularly with conscious patients.¹² Paramedics in the study acknowledged that when the clinical indication was clear, most were willing to push past the awkwardness. But when the reason for an accurate temperature was ambiguous, the discomfort factor often won out, and a less accurate method was chosen instead.

At home, the dynamic is similar. If you are a caregiver taking a rectal temperature on a family member, clear communication helps. Explain why you are using this method rather than an oral or forehead thermometer. Keep the person draped with a sheet or blanket so they are not unnecessarily exposed. The procedure takes under a minute from insertion to reading, so the discomfort is brief. Having everything prepared in advance, with lubricant already on the thermometer tip and gloves already on, reduces the time spent and the awkwardness of fumbling with supplies.

For people who are cognitively impaired, confused, or agitated, rectal thermometry can actually be easier than oral measurement because it does not require the person to cooperate by holding a thermometer under their tongue. But it does require them to hold reasonably still. If someone is actively combative, attempting rectal measurement can cause injury and is not worth the risk. In those cases, a tympanic or temporal artery reading, even with its accuracy limitations, is the safer choice.

Hygiene and Thermometer Selection

If you plan to take rectal temperatures at home, designate a thermometer for rectal use only and label it. Do not use the same thermometer orally and rectally, even with thorough cleaning between uses. Cross-contamination with gut bacteria is a real concern, and labeling prevents mix-ups by other household members.

Standard digital thermometers with a flexible tip work well for adults and are inexpensive. Glass mercury thermometers should not be used. They break, the mercury is toxic, and many jurisdictions have banned their sale. Digital models read in under a minute, and the flexible tip reduces the risk of injury from sudden movement during measurement.

After each use, clean the thermometer with isopropyl alcohol or warm soapy water. If you used a disposable probe cover, discard it immediately. Store the thermometer in its case, away from other household thermometers, and replace it if the tip becomes scratched or the digital display starts acting erratically. A damaged thermometer tip can harbor bacteria in micro-scratches that cleaning will not reach.

Temperature Monitoring in Critical Care

In hospitals, rectal probes are just one option among several for continuous core temperature monitoring. In intensive-care settings where patients are unconscious or sedated, clinicians sometimes use esophageal probes or urinary bladder temperature catheters instead. A study of comatose patients after cardiac arrest found that esophageal temperature responded faster than urinary bladder temperature during therapeutic cooling, with the esophageal site reflecting changes at about 1.1°C per hour compared to about 0.8°C per hour in the bladder.¹³ Both esophageal and bladder sites correlated strongly with each other, but the speed difference mattered when clinicians needed to hit a precise target temperature quickly.

Rectal temperature sits somewhere in between these hospital-grade methods in terms of response speed. It is slower than esophageal but widely accessible without specialized equipment. For the vast majority of situations outside an ICU, including home caregiving, athletic training rooms, and ambulance settings, a simple digital rectal thermometer gives you the most accurate reading you can realistically get. The more invasive monitoring sites exist for patients who are already intubated or catheterized and where second-by-second temperature tracking is part of a treatment protocol, not for general fever assessment.