B-026 Pseudohypobicarbonatemia in a Multiple Myeloma Patient with Hyperproteinemia: A Case Report

Abstract Background Metabolic acidosis is defined by blood pH below 7.32 (venous), resulting from acid accumulation or bicarbonate loss, and classified as high anion gap (HAGMA) or normal anion gap (NAGMA). This case highlights the importance of understanding the limitations of analytical methods in diagnosing metabolic acidosis. A 47-year-old male with IgA lambda multiple myeloma complicated by cast nephropathy and chronic kidney disease presented with hypoxia, dyspnea, and chest pain one day after chemotherapy administration. On admission, his oxygen saturation was 70% due to volume overload in the context of hypertension and acute kidney injury. He received oxygen, antihypertensives, intravenous steroids, and diuretics, which improved his clinical symptoms. Notably, his total serum protein and M-spike were 11.2 g/dL, and 6.5 g/dL, respectively, measured 12 days prior to admission. Laboratory evaluation during this admission showed profound hypobicarbonatemia of 10 mmol/L with HAGMA of 22, total protein of 10.5 g/dL, and monoclonal free lambda light chain of 1170 mg/L. Therefore, the patient was treated with intravenous bicarbonate infusion. The total bicarbonate remained low on the repeat measurement by chemistry analyzer, and the etiology of the hypobicarbonatemia was termed “remained unclear” by the clinical team. The bicarbonate level and pH were later determined with a blood gas analyzer, resulting in values of 7.35 and 22 mmol/L and the bicarbonate therapy was discontinued. His estimated M-spike and total protein decreased after chemotherapy, and both the blood gas and total bicarbonate levels were within the reference range. Methods Two different methods for determining serum bicarbonate were used: the Radiometer ABL800 FLEX blood gas analyzer and the Ortho Clinical Diagnostic VITROS System 5600 ECO2 Slide. The ABL800 FLEX uses direct ion-selective electrodes (ISE) to measure pH and pCO2 and calculate bicarbonate via the Henderson-Hasselbalch equation. The VITROS ECO2 Slide, a dry chemistry tool, measures total carbon dioxide (bicarbonate plus dissolved CO2) with enzyme-driven reactions and reflectance spectrophotometry. Results See Table. Note that due to interference with the Ortho bicarbonate values remain very low despite the normal pH levels. Conclusion This case with hyperproteinemia in a patient with multiple myeloma demonstrates pseudohypobicarbonatemia, which is the first reported case using the Ortho Clinical Diagnostics VITROS series. Pseudohypobicarbonatemia is a condition in which bicarbonate levels in the blood appear falsely low due to analytical interference, such as light absorbance and scattering caused by hyperproteinemia or hyperlipidemia. This occurs in the spectrophotometric method, where the sample reduces the light intensity due to absorption, rather than indicating a true physiological deficiency, potentially leading to confusion for clinicians. Pseudohypobicarbonatemia can be accurately confirmed using direct potentiometry ISE methods on a blood gas analyzer, which measures bicarbonate in whole blood without the need for sample pretreatment or dilution and is unaffected by turbidity or high protein levels. This case highlights the potential for misinterpretation of metabolic acidosis due to discrepancies between laboratory measurement methods. Pseudohypobicarbonatemia can led to unnecessary testing and treatment, and prolong hospital stay. Clinicians should be mindful of the limitations of various testing methods to improve patient management.