The Essentials of Cortisol Saliva Test You Must Know
Cortisol is a steroid hormone produced by the adrenal glands that plays a vital role in regulating metabolism, immune function, blood pressure, and the body's response to stress. It is often referred to as the "stress hormone" since levels fluctuate in response to physical or psychological stress. Cortisol levels follow a circadian rhythm and are typically highest in the morning after waking up and lowest in the evening before bedtime.
Abnormal cortisol levels can indicate disorders of the hypothalamic-pituitary-adrenal (HPA) axis like Cushing's syndrome or Addison's disease. Cushing's syndrome is characterized by excessive cortisol production while Addison's disease is caused by adrenal insufficiency and insufficient cortisol. Testing cortisol levels helps diagnose these conditions and monitor treatment effectiveness. Cortisol testing may use blood, urine, or saliva samples. Salivary cortisol testing is emerging as a preferred method for assessing cortisol rhythms and diagnosing adrenal disorders.
The salivary cortisol test measures the level of free unbound cortisol in saliva, which closely correlates with levels of free serum cortisol. Cortisol passes from the bloodstream into saliva by diffusing through the salivary glands. Salivary cortisol reflects the biological active form of cortisol and is not affected by conditions that alter cortisol protein binding.
Patients are typically provided a commercial saliva collection kit containing tubes, collection swabs, and detailed instructions. Common collection times are first morning upon waking, 30-60 minutes after waking, midday, evening, and late night before bed. The patient places the absorbent swab under the tongue for 1-2 minutes until saturated with saliva. The swab is then placed back in the tube without touching it. Tubes are labeled with patient information and collection time. Patients refrigerate samples until returning to the lab.
At least 0.5-1 mL of clear saliva free of blood or particulates is required. Samples are stable refrigerated for up to a week and frozen for longer periods. Food intake, brushing teeth, smoking, and blood contamination can affect results. Patients should avoid these 30-60 minutes before collecting saliva.
In healthy individuals, salivary cortisol follows a distinct circadian rhythm with peak levels in the early morning up to 20-25 nmol/L and a nadir at midnight of <3.5 nmol/L. Levels then rise steeply in the first hour after waking, peaking around 30-45 minutes after waking at 10-15 nmol/L. This post-waking spike is absent in adrenal insufficiency. Levels gradually decline throughout the day reaching 3-10 nmol/L in the evening.
Reference ranges vary by laboratory but typical norms are:
Abnormal loss of diurnal variation or cortisol levels outside expected ranges can indicate medical conditions affecting the HPA axis.
Elevated nighttime salivary cortisol is a useful screening test for Cushing's syndrome, with midnight samples >145 ng/dL (4 nmol/L) suggesting Cushing's. Midnight salivary cortisol assays have a sensitivity of 96-100% and specificity of 93-100% for Cushing’s. The test is convenient, non-invasive, and avoids hospitalization for midnight serum sampling.
If midnight salivary cortisol is high, confirmatory testing is warranted such as a 24-hour urinary free cortisol, dexamethasone suppression test, or ACTH stimulation test. Salivary testing may also be used to monitor treatment effectiveness in Cushing’s patients. With successful treatment, abnormally high midnight salivary cortisol levels should decrease to normal.
Patients with Addison's disease fail to produce the early morning cortisol spike and high levels. First morning and 30-60 minute post-waking salivary cortisol are typically low. midnight levels may also be low as the normal late-night rise in ACTH drives minimal cortisol secretion in adrenal insufficiency.
The 30-60 minute post-waking salivary cortisol test has a sensitivity and specificity exceeding 90% for diagnosing Addison’s disease. Critics argue the test's accuracy depends on patient adherence with timing and protocols. To compensate, some recommend additional sampling times or combining the first morning sample with a cosyntropin stimulation test.
Several characteristics make salivary cortisol an attractive alternative to traditional serum testing:
Salivary results can be impacted by improper collection technique, blood contamination, and adherence to timing protocols. Optimal diagnostic accuracy requires close patient cooperation. Providers should give clear instructions on collecting samples.
Applications of salivary cortisol testing include:
Clinical interpretation of salivary cortisol requires an understanding of the HPA axis and influences on cortisol secretion. Providers should consider the patient’s symptoms and clinical context when interpreting results. An endocrinologist may be consulted for equivocal findings.
Potential limitations of salivary cortisol testing include:
To optimize accuracy, patients should be carefully instructed on proper technique. Samples with visual contamination or improper collection should be rejected. Providers must account for factors that transiently increase cortisol like stress or exercise when interpreting results. Overall, salivary cortisol offers a convenient outpatient method for assessing adrenal function.
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A Simple Guide to Cortisol Saliva Test Instructions