Interpretation of color change in blood with varying degree of methemoglobinemia.

The ability to detect the characteristic "chocolate" brown color of methemoglobin containing blood was tested in a double blind study. We prepared three concentrations of methemoglobin by adding sodium nitrite to 1-2 ml of venous, heparinized blood that was "arterialized" by mixing with room air in vacutainer tubes. House officers (HO) were asked, in a standardized protocol, to make observations ("chocolate", "abnormal" or "normal") about the methemoglobin blood alone (first in Vacutainer, and then on filter paper), and then in comparison with control blood, again in Vacutainer, then filter paper. Each HO assessed only one methemoglobin concentration. Three methemoglobin blood samples were studied with 12-14%, 26-35% and 68% methemoglobin concentrations respectively. We found that the ability to detect visually an abnormal appearance of methemoglobin was significantly improved when the test sample was compared to control blood. The descriptive term "darker than normal" was much more commonly used than "chocolate" to describe the methemoglobin blood.