Postcollection rise in methemoglobin level in frozen blood specimens

A patient with nonspecific complaints had four previous venous blood samples showing elevated methemoglobin fractions of 15.6-20.1%. Cooximetry on a fresh specimen revealed a methemoglobin fraction of 0.8%, while that reported by the original laboratory on the simultaneously collected specimen was 14.9%. The laboratory assayed the specimen after holding it in frozen storage. Venous blood from a healthy volunteer was assayed by cooximetry after storage under conditions of room temperature (22-24 degrees C), refrigeration (1-4 degrees C), and freezing (-14 to -12 degrees C). Methemoglobin level in frozen-thawed specimens rose over time from 1.8% (0.29 g/dL) at 6 hour to 10.9% (1.71 g/dL) after 6 days. With the exception of a single specimen stored in an EDTA-containing tube at room temperature for 6 days, methemoglobin in nonfrozen specimens never exceeded 0.8% (0.12 g/dL).