Falsely low calcium measurements after high volume plasma exchange in a patient with liver failure

A 63-year-old male with lactic acidosis secondary to fialuridine-induced liver failure underwent seven plasma exchanges while awaiting orthotopic liver transplantation. Following plasma exchange, total serum calcium concentrations measured by conventional clinical chemistry methods were significantly lower than the elemental calciums determined by atomic absorption spectroscopy (P = 0.004). The difference in calcium measured by atomic absorption and by conventional methods correlated with serum citrate concentration (R = 0.77) Following the first exchange, the serum lactic acid concentration decreased from 10.2 to 4.4 mmol/L. These results suggest that plasma exchange may aid in the removal of metabolic products such as lactic acid in patients with liver failure. However, the accumulation of unmetabolized citrate may also result in falsely low total calcium measurements in some patients who undergo plasma exchange.