Potassium but not lactate dehydrogenase elevation due to in vitro hemolysis is higher in capillary than in venous blood samples

Context.-Elevated potassium concentrations due to in vitro hemolysis can lead to errors in diagnoses and treatment. Recently, we observed that potassium elevation in capillary samples appeared higher than expected, based on hemolytic index (H-index).

Objective.-To investigate the relation between potassium increase and H-index for capillary samples. As a control, the same analysis was performed for lactate dehydrogenase (LDH).

Design.-Potassium results of 332 760 venous and 2620 capillary samples were selected. For LDH, 135 974 venous and 999 capillary samples were included. Venous and capillary samples were differentiated by using patient age, as we perform mostly capillary blood sampling in children and venous sampling in adults. Results were obtained with Beckman-Coulter DxC800 analyzers.

Results.-The increase in potassium with increasing H-index was considerably higher for capillary samples than venous samples. Linear regression revealed a potassium increase of 0.38 mEq/L per increment in H-index for capillary samples, whereas a 0.17 mEq/L increase was found for venous samples. For LDH, no differences were found between venous and capillary samples.

Conclusions.-At identical H-index, capillary samples showed higher potassium elevations than venous samples. A possible explanation is that capillary sampling causes increased leakage of ions, such as potassium, from erythrocytes, compared with proteins such as hemoglobin and LDH. These results are especially important considering the increasing use of whole blood point-of-care analyzers, where the H-index is often not determined. Potassium results should therefore be interpreted with caution to avoid severe misdiagnosis of hypokalemia and hyperkalemia. (Arch Pathol Lab Med. 2012; 136: 1262-1265; doi: 10.5858/arpa.2011-0319-OA)