Should You Repeat the CO₂ Test?
Yes, you should repeat the serum CO₂ measurement using a properly collected specimen to confirm whether the low value is real or artifactual. The laboratory has explicitly warned that aliquoted specimens exposed to air can yield falsely low CO₂ readings, making this result unreliable for clinical decision-making 1, 2.
Why the Current Result is Suspect
The lab's comment about specimen aliquoting and air exposure is critical. Research demonstrates that:
- Underfilled collection tubes cause significant CO₂ loss—bicarbonate readings decline progressively as tube fill volume decreases 2
- Air exposure duration matters, with plasma CO₂ dropping by 1.3 mmol/L after just 15 minutes of aliquot exposure 1
- Transport delays compound the problem, with mean reductions of 1.5 mmol/L after 4 hours 1
A CO₂ of 19 mmol/L could easily represent a true value of 21-23 mmol/L (normal range) if specimen handling was suboptimal.
How to Obtain an Accurate Repeat Measurement
To get a reliable result:
- Draw blood directly into a completely filled vacutainer tube (not partially filled—this alone can cause 2-4 mmol/L artifactual decreases) 2
- Minimize air exposure—send the whole blood specimen immediately to the lab without aliquoting
- Request immediate analysis upon arrival at the laboratory
- Consider arterial blood gas (ABG) if you need immediate, definitive results—ABG bicarbonate measurements are not subject to the same air exposure artifacts 3
Real Causes of Low CO₂ (If Repeat Confirms True Low Value)
If the repeat specimen confirms CO₂ <22 mmol/L, you're dealing with metabolic acidosis. Work through this systematically 4:
High Anion Gap Metabolic Acidosis
Calculate anion gap = Na⁺ - (Cl⁻ + HCO₃⁻). If elevated (>12-14):
- Lactic acidosis from tissue hypoxia (sepsis, shock, hypoxemia, low cardiac output states) 4
- Diabetic ketoacidosis or other ketoacidosis
- Renal failure (inability to excrete metabolic acids) 4
- Toxic ingestions: methanol, ethylene glycol, salicylates 5
Normal Anion Gap Metabolic Acidosis
If anion gap is normal:
- Bicarbonate loss from GI tract: chronic diarrhea 4
- Renal tubular acidosis (kidney losing bicarbonate directly) 4
Respiratory Compensation
Check if this is actually chronic respiratory alkalosis with metabolic compensation (rare, but pH and pCO₂ on ABG will clarify).
Critical Pitfall: Pseudohypobicarbonatemia
Before treating, rule out laboratory interference causing falsely low readings 6, 7, 3:
- Severe hypertriglyceridemia (lipid interference with enzymatic assays) 6
- Paraproteinemia (multiple myeloma, amyloidosis interfering with measurement) 7, 3
- Different analyzer methods can give discrepant results 3
If the patient is asymptomatic with no acid-base symptoms and the repeat properly-collected specimen still shows low CO₂, obtain an ABG. A normal pH and bicarbonate on ABG definitively proves pseudohypobicarbonatemia 3.
When to Treat True Metabolic Acidosis
If confirmed real and bicarbonate <22 mmol/L with eGFR <30 mL/min/1.73m², consider oral bicarbonate supplementation to maintain bicarbonate in normal range 8. However, for critically ill patients with shock and severe acidosis (pH <7.30), recent evidence shows sodium bicarbonate infusion does not improve major adverse kidney events or mortality compared to placebo 9, so focus on treating the underlying cause rather than the acidosis itself.
Bottom line: Repeat the test properly before making any clinical decisions. The current result is likely artifactual.