CAR Add-On
The CAR Add-On measures salivary cortisol at waking and 30 minutes later to quantify the cortisol awakening response, a marker of HPA-axis reactivity. Blunted or absent CAR has been associated with burnout, chronic stress, and PTSD, while an exaggerated response may reflect heightened anticipatory stress.
3
Biomarkers
Saliva
Sample
7–10 business days
Turnaround
Yes
Fasting
What is the CAR Add-On test?
The CAR Add-On measures salivary cortisol at awakening and 30 minutes later to quantify the cortisol awakening response, a discrete neuroendocrine phenomenon reflecting the acute activation of the hypothalamic-pituitary-adrenal axis upon waking. Practitioners in integrative and functional-medicine settings order the CAR add-on when standard single-point or four-point diurnal cortisol profiles do not capture the early-morning dynamics suspected to be relevant in a patient with chronic stress, burnout, or unexplained fatigue. The CAR is not part of routine endocrine diagnostics for adrenal insufficiency or Cushing syndrome — serum cortisol, ACTH, and suppression or stimulation tests remain the standard there. Instead, the CAR is used as a functional assessment of HPA-axis reactivity and resilience, interpreted alongside symptoms, sleep quality, and the remainder of the diurnal cortisol curve. Because the CAR's clinical validity and standardisation are still evolving, results guide functional intervention rather than serving as a diagnostic endpoint.
What does the CAR Add-On test measure?
Cortisol secretion follows a circadian rhythm driven by the suprachiasmatic nucleus and the hypothalamic-pituitary-adrenal axis. Upon waking, cortisol concentration rises sharply — a phenomenon termed the cortisol awakening response (CAR) — reaching a peak 30–45 minutes after waking. The awakening sample captures the nadir before this surge begins and anchors interpretation of the subsequent rise.
The baseline awakening cortisol level provides context for the magnitude of the cortisol awakening response. A blunted or absent CAR, where the 30- or 45-minute post-waking value fails to rise meaningfully above the awakening baseline, has been associated with chronic stress, burnout, and certain psychiatric and inflammatory conditions, though the clinical utility of the CAR remains an area of active research.
The post-waking cortisol sample, collected 30 minutes after waking, captures the ascending limb of the cortisol awakening response. This surge is thought to mobilise energy and cognitive resources for the day ahead and is influenced by anticipatory stress, sleep quality, and HPA-axis reactivity.
The magnitude of the increase from waking to 30 minutes post-waking is the primary metric of the CAR. A blunted response may signal HPA-axis exhaustion or dysregulation in the setting of chronic stress, while an exaggerated response has been linked to hypervigilance and anxiety, though findings are inconsistent across studies.
The cortisol awakening response is a derived marker calculated as the difference (or percentage change) between the 30-minute post-waking sample and the awakening baseline. It represents the acute HPA-axis response to the transition from sleep to wakefulness and is sensitive to both psychological state and sleep architecture.
A reduced or absent CAR has been reported in populations with burnout, post-traumatic stress disorder, chronic fatigue, and certain autoimmune conditions, though causality and clinical specificity remain under investigation. An exaggerated CAR may reflect heightened anticipatory stress. The CAR is used as a functional marker in integrative and functional-medicine settings but is not a stand-alone diagnostic tool.
Why is the CAR Add-On test ordered?
- Patients reporting symptoms of burnout, chronic stress, or poor stress resilience in whom a blunted HPA-axis response is suspected
- Investigation of unexplained chronic fatigue or post-exertional malaise where diurnal cortisol rhythm may be disrupted
- Assessment of HPA-axis reactivity in individuals with a history of trauma or prolonged psychosocial stress
- Functional evaluation of morning energy and cognitive function complaints not explained by thyroid, anaemia, or sleep disorders
- Monitoring of integrative or lifestyle interventions aimed at restoring HPA-axis regulation (adaptogenic herbs, stress-management protocols, sleep optimisation)
- Augmentation of a standard four-point diurnal cortisol profile when early-morning dynamics are clinically relevant
- Patients with autoimmune or chronic inflammatory conditions in which HPA-axis dysregulation is hypothesised to contribute to symptom burden
- Screening in high-stress occupations or performance settings (elite athletes, shift workers) where HPA-axis resilience is of interest
Sample collection and turnaround
Sample type
Saliva
Fasting required
Yes
Collection method
At-home collection kit
Turnaround
7–10 business days
Collection notes
Patient collects saliva at home using the provided collection devices. Two samples are required: one immediately upon waking (before getting out of bed, before any food, drink, tooth-brushing, or activity) and a second exactly 30 minutes later. Timing adherence is critical. Patients must not consume food, drink (except plain water), caffeine, or tobacco, and must not brush teeth or engage in physical activity between the two samples. The kit is typically shipped with detailed timing instructions and a log sheet to record exact collection times. Samples are stable at room temperature briefly but should be refrigerated and returned to the laboratory promptly.
Specimen requirements
Two saliva samples collected in the provided tubes (typically passive drool or absorbent device). Minimum volume per manufacturer specification. Patient must record exact waking time and collection times; deviation from the 30-minute interval compromises validity.
What can affect CAR Add-On results?
The CAR is exquisitely sensitive to timing; even a 5–10 minute delay in the post-waking sample can alter the measured response. Non-adherence (rising before sampling, delaying the 30-minute collection) is a major source of variability and false results.
Fragmented sleep, short sleep duration, and sleep-phase disruption can blunt or shift the CAR. Recent sleep deprivation or shift work may confound interpretation.
Anticipation of a stressful event or rumination upon waking can exaggerate the CAR. The psychological state on the morning of collection influences the result.
Systemic glucocorticoids suppress endogenous cortisol, invalidating the test. Oestrogen-containing contraceptives raise cortisol-binding globulin, increasing total but not necessarily free cortisol; salivary cortisol reflects free hormone but interpretation may differ.
Stimulants and exercise acutely raise cortisol. Protocols typically prohibit caffeine, smoking, and exertion until after the 30-minute sample is collected.
Food, drink (except plain water), or blood from gum disease can contaminate saliva and alter measured cortisol concentration. Strict fasting from waking until after the second collection is required.
What do CAR Add-On results mean?
Elevated
An exaggerated CAR (a very large percentage rise from waking to 30 minutes) may be seen with heightened anticipatory stress, anxiety, hypervigilance, or in the early phase of chronic stress before HPA-axis exhaustion develops. It is not a pathological diagnosis but may guide stress-management and anxiolytic interventions.
Normal
A robust CAR (typically ≥50% rise) suggests intact HPA-axis reactivity and normal circadian signalling at waking. It does not exclude other endocrine, metabolic, or psychological contributors to fatigue or stress symptoms, so clinical context remains paramount.
Reduced
A blunted or absent CAR — where the 30-minute sample fails to rise meaningfully above the awakening baseline — has been associated with burnout, chronic fatigue, PTSD, prolonged psychosocial stress, and certain inflammatory and autoimmune conditions. Proposed mechanisms include down-regulation of HPA-axis reactivity or glucocorticoid-receptor resistance. Interpretation should integrate the full diurnal cortisol profile, symptoms, and chronicity of stress exposure.
Note: The CAR is a functional marker with substantial intra-individual variability and no universally accepted diagnostic thresholds. Collection-timing errors are common and can invalidate results. The clinical significance of CAR alterations is still under investigation; it should be used to inform functional treatment approaches rather than as a stand-alone diagnostic tool. Salivary cortisol does not replace serum cortisol or ACTH testing for diagnosis of adrenal insufficiency or Cushing syndrome.
Frequently asked questions
The cortisol awakening response (CAR) is the sharp rise in cortisol that occurs in the first 30–45 minutes after waking, driven by the hypothalamic-pituitary-adrenal axis. It is a distinct phase of the diurnal cortisol rhythm and is sensitive to stress, sleep quality, and HPA-axis regulation.
How to access the CAR Add-On test
The CAR Add-On reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Chida Y, Steptoe A. Cortisol awakening response and psychosocial factors: a systematic review and meta-analysis. Biol Psychol. (2009)
- Clow A, et al. The cortisol awakening response: more than a measure of HPA axis function. Neurosci Biobehav Rev. (2010)
- Stalder T, et al. Assessment of the cortisol awakening response: expert consensus guidelines. Psychoneuroendocrinology. (2016)
- Pruessner JC, et al. Two formulas for computation of the area under the curve represent measures of total hormone concentration versus time-dependent change. Psychoneuroendocrinology. (2003)
- Miller GE, et al. If it goes up, must it come down? Chronic stress and the hypothalamic-pituitary-adrenocortical axis in humans. Psychol Bull. (2007)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.