A case of factitious hyponatremia and hypokalemia due to the presence of fibrin gel in serum.

We describe here the case of a 84-year-old woman under warfarin therapy for atrial fibrillation, who was admitted to the Emergency Department for severe gluteal hemorrhage. Blood samples were drawn after patient admission and transported within 20 min to the laboratory. Results of laboratory testing revealed a markedly increased value of international normalized ratio and extremely low values of serum sodium (60 mmol/L) and potassium (1.4 mmol/L). A second serum sample was collected approximately 2 h after the former, showing normal values of both sodium (145 mmol/L) and potassium (3.5 mmol/L). The analysis of the specimen collected upon patient admission revealed the presence of a small, transparent fibrin gel in the serum, which was partially aspirated by the sample probe of the instrument. This case underscores the importance that serum samples need to be allowed to clot completely prior to centrifugation and analysis, especially those collected from patients under anticoagulant treatment.