Ask the intern

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An interesting case was reported in Nephrology this month that highlights the importance of questioning whether or not a lab result is true. This is a very important skill for all internists but particularly nephrologists who are dependent on lab results for most of our practice. A woman with a history of Still’s disease was admitted to hospital with a low grade fever and joint swelling. Her only meds were low dose prednisone and diclofenac (an NSAID). Her labs on admission showed:

Na 170 mmol/L, K 3.2 mmol/L, low-normal urea and creatinine. Her other labs were normal. She denied any thirst and did not look volume depleted. Because of the doubt about the accuracy of the sample, it was repeated. Her repeat sodium was 139mmol/L.

After questioning the intern that took the labs, it became apparent that multiple attempts had been made to take blood. The intern had managed to take a coagulation sample but had failed to get a sample for electrolytes. As a result, she transferred some of the blood from the coag bottle to the lithium heparin bottle. The coag bottle contained trisodium citrate and this had contaminated the serum sample resulting in a spuriously high sodium concentration. This was confirmed by testing the original and repeat sample for calcium. In the original sample it was 1.88 mmol/L and in the repeat it was 2.34 mmol/L. This was due to the chelation of calcium by citrate. Classically, spurious hyperkalemia and hypocalcemia occur in the setting of contamination by EDTA. Pseudohypernatremia is less common but should be considered first in cases of severe asymptomatic hypernatremia. 

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