Screening Serum Bicarbonate Level for OHS
A serum bicarbonate level of ≥27 mmol/L (or >27 mmol/L) is the screening threshold used to suggest obesity hypoventilation syndrome (OHS) in patients with low to moderate pretest probability (<20%). 1
Clinical Application Algorithm
The 2019 American Thoracic Society guidelines provide a clear, risk-stratified approach:
For Low to Moderate Risk Patients (Pretest probability <20%)
- BMI typically 30-40 kg/m², or BMI >35 kg/m² without high-risk features
- If serum bicarbonate <27 mmol/L: OHS is very unlikely; no arterial blood gas needed 1
- If serum bicarbonate ≥27 mmol/L: Proceed to measure PaCO₂ via arterial blood gas to confirm or rule out OHS 1
This threshold allows clinicians to avoid arterial blood gas testing in 64-74% of obese patients with OSA who have bicarbonate <27 mmol/L, while identifying the 26-36% who need confirmatory testing 1.
For High Risk Patients
Patients with severe obesity plus typical signs/symptoms (dyspnea, nocturia, lower extremity edema, excessive daytime sleepiness, fatigue, loud snoring, witnessed apneas) and/or mild awake hypoxemia or significant sleep hypoxemia should proceed directly to PaCO₂ measurement rather than relying on bicarbonate screening 1.
Evidence Quality and Nuances
Important caveats about the 27 mmol/L threshold:
- The guideline acknowledges very low certainty of evidence for this recommendation 1
- Research confirms bicarbonate ≥27 mmol/L has moderate sensitivity (54-77%) and specificity (75-80%) for OHS 2
- Recent data shows bicarbonate has only moderate diagnostic accuracy (AUC 0.70-0.92 depending on outcome) 3, 4
- Normal bicarbonate does not rule out sleep hypoventilation - sensitivity is only 54-61% at the 27 mmol/L cutoff 4
The panel chose 27 mmol/L because: (1) serum bicarbonate is readily available on routine chemistry panels, (2) it reduces unnecessary arterial punctures, and (3) any missed OHS cases at bicarbonate <27 mmol/L likely have only mild hypercapnia 1.
Supporting Research Findings
Multiple studies validate this approach:
- In bariatric surgery cohorts, OHS prevalence ranges from 19.5% to 68.5% depending on diagnostic criteria 5, 6, 7
- Bicarbonate ≥27 mmol/L combined with nadir SpO₂ <80% captures most OHS cases, missing only 3 of 64 patients in one study 2
- Independent predictors of OHS include elevated bicarbonate, neck circumference, nocturnal hypoxia, and AHI severity 5, 2, 3
What NOT to Use
Do not use SpO₂ alone for screening decisions until more data becomes available - the ATS explicitly recommends against this due to insufficient evidence 1.