Sleep Apnea

Blood tests related to sleep apnea

What is sleep apnea?

Risk factors and causes

Health consequences of untreated sleep apnea

Diagnosis: the sleep study (polysomnography)

Blood tests cannot diagnose sleep apnea. The definitive test is a polysomnogram (sleep study): performed overnight in a sleep clinic or via a home sleep apnea test (HSAT). It records breathing effort, airflow, oxygen saturation, brain waves, heart rate and limb movements. The Apnea-Hypopnea Index (AHI) measures severity: mild (5–14 events/hour), moderate (15–29 events/hour) or severe (≥30 events/hour).

Questions to ask your doctor

• Should I have a home sleep test or an in-lab sleep study?
• Could my thyroid be contributing to my sleep apnea?
• What is my AHI score and what severity does that indicate?
• Is CPAP the right treatment for me, or are there alternatives?
• How does sleep apnea affect my heart and diabetes risk?

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Chronic Insomnia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK526136

References

Sources cited on this page. PubMed links open the original abstract.

1. Young T, Palta M, Dempsey J, Skatrud J, Weber S, Badr S. The occurrence of sleep-disordered breathing among middle-aged adults. N Engl J Med. 1993;328(17):1230–1235. PMID 8464434 · doi:10.1056/NEJM199304293281704

Obstructive versus central sleep apnoea – two different mechanisms

Obstructive sleep apnoea (OSA) – the far more common form, affecting 4% of men and 2% of women – is caused by the collapse of pharyngeal soft tissue (tongue, soft palate, uvula) during sleep. When the upper airway muscles relax, airflow ceases despite continued thoracic effort. Central sleep apnoea (CSA) is caused by failure of the respiratory drive – the brain fails to send the signal to breathe. CSA occurs in heart failure (Cheyne-Stokes respiration), brainstem disorders, and opioid use. Treatment differs fundamentally: CPAP is first-line for OSA; CSA requires management of the underlying condition and may require adaptive servo-ventilation (ASV – contraindicated in heart failure with EF below 45% after SERVE-HF data).

Blood tests in sleep apnoea workup

Sleep apnoea is diagnosed by sleep studies (polysomnography or home respiratory monitoring) rather than blood tests. However, blood tests identify metabolic consequences and comorbidities that modify treatment priority:

• HbA1c and fasting glucose: OSA and type 2 diabetes are strongly associated – 70–80% of obese type 2 diabetics have OSA, and intermittent nocturnal hypoxaemia independently worsens insulin resistance. Effective CPAP treatment modestly reduces HbA1c (approximately 0.3–0.5 mmol/mol) in diabetics with OSA.
• TSH: Hypothyroidism causes weight gain (worsening OSA mechanically) and also directly reduces pharyngeal muscle tone. Untreated hypothyroidism can both cause and dramatically worsen OSA – thyroid replacement sometimes significantly improves OSA severity.
• FBC: Secondary polycythaemia (raised haemoglobin and haematocrit) occurs when chronic nocturnal hypoxaemia stimulates erythropoietin production – the body's attempt to increase oxygen-carrying capacity. An elevated haematocrit (above 55% in men, 50% in women) in an obese snorer supports significant hypoxaemia and urgent sleep study referral.
• Lipid panel and cardiovascular risk markers: OSA independently increases cardiovascular risk via intermittent hypoxaemia, sympathetic activation, oxidative stress, and systemic inflammation. CRP, NT-proBNP, and lipid panel establish cardiovascular risk at diagnosis – CPAP reduces cardiovascular events in those with OSA.

The STOP-BANG questionnaire

The STOP-BANG screening tool identifies patients at high risk for OSA who need sleep studies. One point each for: Snoring (loudly), Tired (daytime sleepiness), Observed apnoeas (witnessed breathing cessation), blood Pressure (hypertension), BMI above 35, Age above 50, Neck circumference above 40 cm (men) / 38 cm (women), Gender (male). Score 5–8 = high risk; 3–4 = intermediate; 0–2 = low. High-risk patients with symptoms warrant referral for oximetry or full polysomnography.