Add-On: CAR
The CAR test measures salivary free cortisol at waking and three subsequent time-points in the first hour, capturing the dynamic surge that reflects HPA-axis reactivity. It is ordered as an add-on to diurnal profiles in patients with chronic stress, burnout, or fatigue to assess awakening-phase responsiveness, interpreted as a functional marker alongside symptoms and clinical context.
1
Biomarkers
Saliva
Sample
7–10 business days
Turnaround
No food, drink, or tooth-brushing before the first sample
Fasting
What is the CAR test?
The Cortisol Awakening Response (CAR) test measures salivary free cortisol at multiple time-points in the first hour after waking, capturing the dynamic surge that reflects HPA-axis reactivity to the transition from sleep to wakefulness. It is ordered as an add-on to baseline diurnal cortisol profiles when practitioners wish to assess not only the overall circadian pattern but also the acute stress-responsiveness of the axis in patients with symptoms of chronic stress, burnout, or fatigue. Unlike random or single-timepoint cortisol, CAR isolates a specific neuroendocrine event and provides insight into how the axis mobilises in anticipation of daily demands. It is used in integrative and functional-medicine settings to guide personalised interventions around sleep hygiene, stress resilience, and circadian rhythm support, though it remains a research-informed rather than guideline-mandated tool and is not a substitute for formal endocrine testing when pathology is suspected.
What does the CAR test measure?
The cortisol awakening response is a specialised manifestation of the hypothalamic–pituitary–adrenal axis, characterised by a sharp rise in free cortisol concentration in the first 30–60 minutes after waking. It is thought to prepare the organism for the anticipated demands of the day and is modulated by sleep quality, light exposure, chronic stress, and central regulation distinct from the basal circadian rhythm. The response is measured in saliva, which reflects the bioactive free fraction of cortisol.
CAR is used as a marker of HPA-axis reactivity and has been studied in relation to chronic stress, burnout, post-traumatic stress disorder, depression, and fatigue syndromes. A blunted or absent awakening response may suggest HPA-axis downregulation or exhaustion, while an exaggerated response can be seen in some anxiety states and early stress adaptation. It is a functional biomarker rather than a diagnostic one and is interpreted in clinical context alongside symptoms and diurnal cortisol patterns.
Why is the CAR test ordered?
- Assessment of HPA-axis reactivity in patients with chronic stress, burnout, or exhaustion not explained by routine endocrine tests
- Investigation of fatigue, low morning energy, or difficulty transitioning to wakefulness despite adequate sleep duration
- Patients with suspected HPA-axis dysregulation who have a flat or abnormal diurnal cortisol profile and require finer characterisation of the awakening phase
- Monitoring of stress-management, mindfulness, or adrenal-support interventions where a dynamic marker of axis reactivity is desired
- Exploration of neuroendocrine correlates in patients with depression, anxiety, post-traumatic stress disorder, or chronic pain syndromes
- Athletes or high-demand professionals with symptoms of overtraining or persistent non-restorative sleep
- Integration with salivary diurnal cortisol when a comprehensive picture of circadian and awakening dynamics is needed
Sample collection and turnaround
Sample type
Saliva
Fasting required
No food, drink, or tooth-brushing before the first sample
Collection method
At-home collection kit
Turnaround
7–10 business days
Collection notes
Patient collects saliva at home using the Genova kit at four time-points: immediately upon waking (before getting out of bed), then at +15, +30, and +45 or +60 minutes (confirm exact protocol in the kit instructions). Samples must be taken before eating, drinking (except water), brushing teeth, or exposure to significant light. Patient records exact collection times and returns specimens by courier per kit instructions.
Specimen requirements
Saliva collected into Salivette or similar passive-drool device at each time-point; store at room temperature or refrigerate and ship with ice pack. Exact protocol and preservative details are kit-specific.
What can affect CAR results?
CAR requires collection immediately upon waking (before rising from bed) and at precise intervals thereafter; delays, movement, food intake, or caffeine before sampling invalidate the profile.
The response is contingent on natural awakening from sleep; alarm clocks, poor sleep, or shift work can alter the magnitude and timing of the CAR.
Bright light at awakening amplifies the cortisol rise, so indoor versus outdoor waking conditions and season may influence results.
Systemic corticosteroids suppress the endogenous axis; oral oestrogen raises cortisol-binding globulin, altering total but not necessarily free cortisol, though cross-reactivity in some assays may occur.
Inflammatory stress and infection activate the HPA axis and can produce an exaggerated or dysregulated awakening response.
Anxiety about the test itself or unusual stressors on the morning of collection can acutely amplify the CAR, so practitioners should contextualise outlier results.
What do CAR results mean?
Elevated
An exaggerated CAR (steep or prolonged rise) may reflect heightened HPA reactivity seen in acute stress adaptation, anxiety disorders, or anticipatory stress. It is not diagnostic of a disease but signals that the axis is mobilising vigorously; clinical correlation with symptoms and psychological state is essential.
Normal
A moderate, physiological rise in cortisol peaking 30–45 minutes after waking indicates healthy HPA reactivity and circadian entrainment. It supports that the axis is responding appropriately to the sleep–wake transition.
Reduced
A blunted or absent CAR — little or no rise from waking — may suggest HPA-axis downregulation, chronic stress exhaustion, or burnout. It has been associated with fatigue syndromes, PTSD, and depression, though it is not specific and requires integration with the broader clinical picture and diurnal profile.
Note: CAR is a functional, context-sensitive marker, not a diagnostic test for adrenal insufficiency or Cushing syndrome. Collection compliance is critical — non-adherent sampling produces artefactual profiles. Results must be interpreted alongside symptoms, diurnal cortisol, and clinical context, and practitioners should frame findings as exploratory rather than definitive.
Frequently asked questions
The cortisol awakening response is the sharp rise in free cortisol that occurs in the first 30–60 minutes after waking, driven by the HPA axis preparing the body for the demands of the day. It is measured in saliva to reflect bioactive cortisol.
How to access the CAR test
The CAR 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.
Create Free AccountAbout this page
Published: October 7, 2026
Last reviewed: October 7, 2026
- Stalder T, et al. Assessment of the cortisol awakening response: expert consensus guidelines. Psychoneuroendocrinology. (2016)
- 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)
- Fries E, et al. A new view on hypocortisolism. Psychoneuroendocrinology. (2005)
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.