Salivary Cortisol Testing -- Single Versus Multiple Daily Tests
Salivary cortisol testing is the preferred clinical method for measuring human cortisol levels. It has several obvious advantages over blood tests, such as greater validity for tissue levels of cortisol, the convenience of home testing, non-invasiveness, and zero risks of infection. But arguably one of the most important features it offers is the ability to take multiple test samples during the same day.
Although urine can also be sampled more than once a day, the constant supply of saliva versus the intermittent supply of urine allows salivary cortisol levels to be taken on-demand at any time of the day or night.
The rhythmic circadian fluctuations of cortisol secretion make it virtually impossible to accurately determine cortisol levels by taking a single sample. Cortisol levels can change dramatically within an hour, and it is how they change over a 24 hour period that often yields the most clinical insight. For example, a common pattern seen in people under stress is above normal cortisol in the morning (8:00 AM), which drops to low or below normal by noon and remains low throughout the rest of the day. A single 8:00 AM sample is in no way an accurate indicator of cortisol levels for other parts of the day or of the pattern of change. Cortisol concentration levels vary 6-8 times in magnitude during a normal 24 hour period. Taking a single sample at the beginning of the day, which is the most common time for single samples to be taken, can be so misleading that it often results in the inappropriate interpretation of the overall cortisol status and consequent therapy for the patient.
The following example illustrates the clinical implications of testing cortisol only once compared to testing multiple times on a single day.
Mary goes to her physician with a number of symptoms including early morning and mid-afternoon fatigue and mild depression. He decides to test her cortisol levels and has an early morning sample taken, unaware that she is not sleeping well and that cortisol can be elevated in the morning due to the stress of restless sleep the night before. When he receives Mary’s test results showing elevated morning cortisol, he provides treatment to flatten her cortisol levels because he views her elevated cortisol as a health risk. Mary follows the treatment as directed, but feels worse. She returns a few weeks later complaining that her symptoms are increasing in duration and intensity and that she has developed new ones as well. Thinking he is not doing enough to flatten her cortisol, the physician doubles her dosage.
Mary gets even worse and consults another physician in her area. Her new physician also says he expects her adrenals are involved in her health problems. After a quick glance at her single sample test result, he tells her he wants to run the salivary cortisol test again, but this time he wants to test her cortisol levels 4 times on the same day.
The results show that even though her morning cortisol level was elevated, as it was in her previous test, the other three cortisol levels were actually low. Indeed, viewing the multiple samples taken on the same day, Mary sees that she is not suffering from an excess of cortisol, but in actuality has low cortisol during most of the day.
As you can see, the clinical implications of using multiple samples rather than a single sample cortisol test are significant. The physician who used the 8:00 am cortisol level as the sole laboratory indicator of Mary’s condition misdiagnosed her and prescribed the wrong treatment. If he had been aware of the circadian variation of cortisol and the consequent importance of looking at the pattern of change, he would have used a more accurate test strategy and achieved a much better outcome for his patient.
Health care practitioners who understand that cortisol normally fluctuates throughout the day routinely order multiple cortisol samples to be taken over a single day. The most common pattern is four times per day at 8:00 am-noon, 4:00 pm, and before bed.
In summary, cortisol has large daily fluctuations from its peak to its nadir. Because of this normal variation, the use of multiple salivary cortisol tests over a single day is the most accurate way to gain insights into the daily fluctuating cortisol pattern of patients and arrive at a proper diagnosis and treatment protocol. The astute health care practitioner uses laboratory tests that most accurately reflect the underlying physiology of the body system(s), they are investigating. Cortisol testing is no exception.