Interpreting Bicarbonate Level Differences in ABG and Serum

Understanding Measurement Differences

The American Thoracic Society recommends considering serum bicarbonate values more reliable for screening purposes, with a threshold of >27 mmol/L indicating the need for ABG confirmation of hypercapnia 1, 2, 3
Serum bicarbonate on a Basic Metabolic Panel (BMP) is actually measured as total CO2 content, which includes bicarbonate (HCO3-), dissolved CO2, and other minor carbon dioxide components, with bicarbonate representing approximately 96% of the total 4

For screening purposes, such as for Obesity Hypoventilation Syndrome, serum bicarbonate levels are more reliable, with a threshold of >27 mmol/L indicating the need for ABG confirmation 1, 2, 3
A serum bicarbonate <27 mmol/L has a very high negative predictive value (99.0%; 95% CI, 97.9–99.6%) for ruling out conditions like OHS, potentially eliminating the need for arterial blood sampling 1, 5

The term "serum bicarbonate" is not strictly accurate because the chemical methods measure all CO2 liberated from the serum, with some laboratories using the more correct term "total serum CO2" 6

Sequential rather than simultaneous sampling can lead to differences, especially in unstable patients, and verifying the timing of sample collection is crucial 7

If verified, the elevated bicarbonate suggests chronic respiratory acidosis with renal compensation, as the kidneys respond to chronic respiratory acidosis by increasing serum bicarbonate level 6
A serum bicarbonate >27 mmol/L in obese patients should prompt consideration of arterial blood gas measurement to confirm hypercapnia 7

The American Journal of Kidney Diseases recommends treatment of elevated serum bicarbonate levels should be directed at identifying and addressing the underlying cause, with correction of metabolic alkalosis through specific interventions based on etiology, while maintaining serum bicarbonate at appropriate levels (22-26 mmol/L for most patients) 8, 9
The American Journal of Kidney Diseases suggests monitoring serum bicarbonate levels monthly in patients with chronic kidney disease (CKD) or on maintenance dialysis 9
For patients with chronic kidney disease and low serum bicarbonate (<22 mmol/L), the American Journal of Kidney Diseases recommends oral bicarbonate supplementation to maintain normal range 8, 10
The American Journal of Respiratory and Critical Care Medicine recommends treating the underlying respiratory condition (COPD, obesity hypoventilation syndrome) for patients with respiratory acidosis with compensation 11
The American Journal of Respiratory and Critical Care Medicine suggests considering sleep study and appropriate respiratory support for patients with suspected obesity hypoventilation syndrome when bicarbonate is >27 mmol/L 11
Critical Care Medicine recommends reserving bicarbonate therapy for severe acidosis (pH <7.1) with acute kidney injury where evidence suggests potential benefit 12

Arterial blood gas analysis alone may be misleading in estimating the internal milieu of myocardial and cerebral intracellular acid-base status, as noted by the British Journal of Anaesthesia 13, 14

The American Thoracic Society recommends a serum bicarbonate threshold of 27 mmol/L to decide when to measure PaCO2 in patients with suspected conditions like Obesity Hypoventilation Syndrome, as reported in the American Journal of Respiratory and Critical Care Medicine 15

The British Medical Association suggests that sodium bicarbonate administration should be limited to patients with severe acidosis (arterial pH <7.1 and base deficit <-10) and special circumstances such as hyperkalaemia or tricyclic antidepressant overdose, according to the British Journal of Anaesthesia and BMJ 13, 14, 16
The British Journal of Anaesthesia notes that potential adverse effects of sodium bicarbonate administration include alkalaemia, hyperosmolarity, and carbon dioxide production 13, 14
The British Journal of Anaesthesia recommends that the best method of reversing acidosis associated with cardiac arrest is to restore spontaneous circulation rather than administering buffers 13, 14

ABG‑derived calculated bicarbonate should be used for immediate metabolic evaluation in critically ill patients who require rapid assessment of pH, PaCO₂, and metabolic status (e.g., patients in shock or with systolic blood pressure < 90 mmHg)【17】.
When patients are at risk for hypercapnic respiratory failure, ABG‑derived bicarbonate is recommended to guide oxygen‑therapy titration【18】【17】.

Normal peripheral oxygen saturation (SpO₂) does not rule out significant acid‑base disturbances or abnormal bicarbonate levels; clinicians should not rely on pulse oximetry alone for acid‑base screening【17】.

A serum bicarbonate (total CO₂) level > 27 mmol/L on a basic metabolic panel should trigger arterial blood gas confirmation to screen for obesity hypoventilation syndrome【19】【20】.
Applying this serum‑bicarbonate threshold can avoid unnecessary arterial puncture in approximately 64 %–74 % of obese patients being evaluated for sleep‑disordered breathing【19】【20】.

Patients with chronic hypercapnia (e.g., COPD) require ABG monitoring after each adjustment of supplemental oxygen to detect worsening respiratory acidosis【18】【21】.

All statements are based on peer‑reviewed evidence and include the cited reference IDs.