What is the expected arterial partial pressure of carbon dioxide (pCO₂) in a patient with serum glucose 400 mg/dL, sodium 144 mEq/L, chloride 113 mEq/L, pH 7.38, and bicarbonate 11 mEq/L?

Medical Advisory BoardAll articles are reviewed for accuracy by our Medical Advisory Board
Educational purpose only • Exercise caution as content is pending human review
Article Review Status
Submitted
Under Review
Approved

Last updated: February 9, 2026 • View editorial policy

Personalize

Help us tailor your experience

Which best describes you? Your choice helps us use language that's most understandable for you.

Expected pCO2 Calculation in Metabolic Acidosis

The expected pCO2 for this patient is approximately 26 mmHg, calculated using the simple formula: pCO2 = HCO3 + 15 (11 + 15 = 26 mmHg).

Clinical Context

This patient presents with:

  • High anion gap metabolic acidosis (Anion gap = 144 - (113 + 11) = 20 mEq/L) 1
  • Appropriate respiratory compensation (pH 7.38 with HCO3 11 mEq/L indicates chronic adaptation) 2
  • Diabetic ketoacidosis (glucose 400 mg/dL with metabolic acidosis) 3

Calculation Methods for Expected pCO2

Primary Formula (Simplest and Most Accurate)

Use: pCO2 = HCO3 + 15

  • This formula has been validated in hemodialysis patients with metabolic acidosis and shows the lowest prediction error (1.7 mmHg RMSE) 1
  • For this patient: 11 + 15 = 26 mmHg expected pCO2 1
  • This formula is interchangeable with the common practical rule but requires no calculations or assumptions 1

Alternative Validated Formulas

Winter's Formula: pCO2 = (1.5 × HCO3) + 8

  • For this patient: (1.5 × 11) + 8 = 24.5 mmHg 1, 3
  • This formula approximates the relationship between pCO2 and bicarbonate in metabolic acidosis 3
  • However, it shows larger prediction errors compared to the simpler formula 1

Common Practical Rule: ΔpCO2 = 1.2 × ΔHCO3

  • Reduction in pCO2 equals 1.2 times the reduction in bicarbonate from normal (24 mEq/L) 1, 4
  • For this patient: 1.2 × (24 - 11) = 15.6 mmHg reduction from baseline 40 mmHg = 24.4 mmHg 1
  • Shows same low error as the simple formula (1.7 mmHg) but requires more computation 1

Fulop's Rule: pCO2 = two digits after pH decimal point

  • For pH 7.38, expected pCO2 would be 38 mmHg 3
  • This rule works best for pH 7.10-7.37 in diabetic ketoacidosis 3
  • This formula is associated with large prediction errors and should be avoided 1

Determining Appropriate Compensation

Expected pCO2 reference range = calculated value ± 3.4 mmHg (2 × RMSE)

  • For this patient: 26 ± 3.4 = 22.6 to 29.4 mmHg 1
  • If measured pCO2 falls outside this range, suspect a superimposed respiratory acid-base disorder 1
  • pCO2 higher than expected suggests concurrent respiratory acidosis 1
  • pCO2 lower than expected suggests concurrent respiratory alkalosis 1

Critical Clinical Pitfalls

Do not confuse this with chronic respiratory acidosis:

  • A pH >7.35 with elevated pCO2 and HCO3 indicates chronic respiratory compensation, not metabolic acidosis 2
  • This patient's pH 7.38 with LOW HCO3 (11 mEq/L) confirms metabolic acidosis, not respiratory pathology 2

Avoid using Fulop's rule in this pH range:

  • Fulop's rule becomes unreliable above pH 7.10-7.15 and would significantly overestimate expected pCO2 in this case 1, 3

Remember this is diabetic ketoacidosis:

  • The high anion gap (20 mEq/L) with hyperglycemia confirms ketoacid accumulation 3
  • Respiratory compensation through hyperventilation is expected and protective 4

References

1
Research

Evaluation of the expected ventilatory response to metabolic acidosis in chronic hemodialysis patients.

Hemodialysis international. International Symposium on Home Hemodialysis, 2018

2
Guideline

Chronic Compensated Respiratory Acidosis Management

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

Related Questions

Is CO2 (carbon dioxide) in a complete metabolic panel (CMP) equivalent to bicarbonate (HCO3-)?
Does a carbon dioxide (CO2) level of 19 indicate acidosis?
What is acidity in medical terms and how is it treated?
In a patient with serum glucose 400 mg/dL, sodium (Na⁺) 144 mEq/L, chloride (Cl⁻) 113 mEq/L, arterial pH 7.38, partial pressure of carbon dioxide (pCO₂) 25 mm Hg, and bicarbonate (HCO₃⁻) 11 mEq/L, which acid‑base disorder is present?
In a patient with serum glucose 400 mg/dL, sodium 144 mEq/L, chloride 113 mEq/L, pH 7.38, partial pressure of carbon dioxide (pCO₂) (value not shown), and bicarbonate (HCO₃⁻) 11 mEq/L, what is the acid‑base disturbance?
How should I order HIV fourth‑generation testing, VDRL (syphilis) screening, and tuberculosis work‑up (sputum smear/GeneXpert, chest X‑ray, and IGRA/TST) for a high‑risk patient?
What is the safest sleep aid for a 72-year-old man in the intensive care unit after open‑heart surgery who is on extracorporeal membrane oxygenation?
What causes a low BUN/creatinine ratio (~7) with low serum phosphorus, high‑normal albumin, and elevated red blood cell count, hemoglobin, and hematocrit?
When should antiseizure medication be initiated in a child who has had a single non‑febrile seizure with a normal electroencephalogram and normal brain magnetic resonance imaging?
How should I evaluate and manage a patient with an absolute lymphocyte count of 495 cells/µL?
What is the recommended management for a patient whose skin biopsy shows an adnexal carcinoma?

Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

Have a follow-up question?

Our Medical A.I. is used by practicing medical doctors at top research institutions around the world. Ask any follow up question and get world-class guideline-backed answers instantly.

Ask Question