Diagnostic Evaluation of Obesity Hypoventilation Syndrome (OHS)

Core Diagnostic Requirements

OHS is defined by the simultaneous presence of (1) obesity (BMI > 30 kg/m²), (2) sleep‑disordered breathing, and (3) awake daytime hypercapnia (PaCO₂ > 45 mm Hg at sea level)【1】【3】【4】.

Home sleep apnea tests (HSAT) can detect obstructive sleep‑apnea patterns and calculate a respiratory event index (REI), but they cannot measure awake arterial blood gases (PaCO₂) and therefore cannot alone confirm OHS【5】【4】.
The American Academy of Sleep Medicine states that HSAT devices lack the capability to detect sleep‑related hypoxia indicative of underlying pulmonary disease; interpretation must be performed by a board‑certified sleep‑medicine physician【5】.

In obese patients with a high pre‑test probability of OHS, direct measurement of PaCO₂ is recommended rather than reliance on surrogate markers【6】【1】.
In patients with low‑to‑moderate probability (< 20 %), serum bicarbonate can be used as a screening tool:
- Bicarbonate < 27 mmol/L makes OHS very unlikely, allowing omission of arterial blood‑gas analysis【4】.
- Bicarbonate ≥ 27 mmol/L warrants arterial blood‑gas measurement; this threshold yields a sensitivity of ≈ 86 % and specificity of ≈ 90 % for detecting OHS【4】.

Full‑night polysomnography or sleep‑respiratory polygraphy is required to characterize the pattern of sleep‑disordered breathing (obstructive vs. non‑obstructive hypoventilation) and to guide therapy【1】【2】【8】.
Whether performed in‑laboratory or via validated home‑based portable monitoring, the sleep study alone does not diagnose OHS; it only identifies the sleep‑disordered breathing component【1】.

Arterial blood‑gas measurement during wakefulness is mandatory to confirm OHS by documenting PaCO₂ > 45 mm Hg【6】【1】【3】.

Approximately 90 % of patients with OHS have concurrent obstructive sleep apnea, and about 70 % of these exhibit severe OSA (AHI > 30 events/h)【1】【8】.

Do not rely solely on home sleep‑study findings to exclude OHS, even when severe OSA is identified.
Do not use awake SpO₂ alone to decide when to measure PaCO₂, as evidence supporting this approach is insufficient【2】【7】.
Arterial blood‑gas measurement remains the essential, non‑negotiable test for OHS diagnosis, with critical implications for mortality, morbidity, and treatment decisions【6】【1】【3】【4】【9】.