Sheep in Wolf's Clothing: Pseudohypobicarbonatemia in a Patient with Multiple Myeloma

Introduction: There are multiple methods to measure serum bicarbonate level which is crucial for diagnosis and management of acid-base disturbances. The most common method is directly measuring the total serum carbon dioxide (CO2) concentration in basic metabolic panel (BMP) via a method based on phosphoenolpyruvate carboxylase (PEPC); another method is by indirect calculation of the bicarbonate concentration via the Henderson-Hasselbalch equation using the measured pH and partial pressure of CO2 (pCO2) in plasma. Multiple confounders can cause discrepancies in the measured bicarbonate in each method, such as increased concentration of serum proteins that can affect the PEPC assay leading to spuriously low bicarbonate values. Case Description: A 47 year old male patient with multiple myeloma, chronic kidney disease (CKD) and hypertension who presented with hypoxia due to pneumonia and volume overload in the setting of hypertension and acute kidney injury on top of CKD. Patient improved after antibiotics, intravenous (IV) diuresis, and blood pressure control. He was noted to have significantly low total CO2 level in BMP on admission to 10 mmol/L with anion gap (AG) of 23 mmol/L. Patient was thought to have high anion gap metabolic acidosis and was treated with an IV bicarbonate infusion. Patient had a normal lactate, normal blood sugar, and no apparent contributing factor that would explain this acid-base disturbance. A simultaneous venous blood gas (VBG) showed a pH of 7.35, pCO2 of 42 mmHg and bicarbonate of 22 mmol/L. IgA level and lambda light chain were both significantly elevated at 3804 mg/dL and 1788.97 mg/L respectively. The discrepancy of bicarbonate level between VBG and BMP was explained by increased paraproteins, which have resulted in artifactual errors in PEPC-based laboratory analysis of serum HCO3. The patient’s AG and accuracy of serum bicarbonate level measurement improved as their IgA and lambda light chain levels subsequently decreased. Discussion: Interpretation of serum CO2 is usually accurate; rarely, there are factors that can cause false readings. Arterial or venous blood gas bicarbonate value can guide you to detect pseudohypobicarbonatemia in the appropriate clinical setting.