Carboxyhemoglobin reference range

Carboxyhemoglobin (COHb) is the product of the reaction between hemoglobin and carbon monoxide, and measurement of COHb is used in the diagnosis of carbon monoxide poisoning. Since carbon monoxide is a common pollutant present in cigarette smoke and car exhaust, it is difficult to be too dogmatic about what constitutes a normal COHb. 

Clearly, a non-smoker living and working in the countryside will have a lower COHb than a cigarette-smoking city slicker, who spends much of his or her working day sitting in slow-moving car traffic. This difficulty has led to the adoption of COHb reference ranges that are frankly wrong; a problem that has been highlighted recently by two US experts in the letter pages of the journal Clinical Chemistry. 

Best evidence quoted by the experts suggests that the upper limit of normal COHb should be set at between 2 and 3 % for non-smokers and between 7 and 9 % for non-smokers.

During an investigation of carbon monoxide poisoning in Florida they identified 2 of 10 hospitals where the upper limit of normal COHb for non-smokers was quoted as 20 %. Adoption of this reference range would result in failure to identify many people (those whose measured COHb is between 3 and 20 %) who might well be suffering carbon monoxide poisoning. 

The experts cite a 1995 survey of 23 Boston hospitals that revealed a wide variation in COHb reference ranges, with 38 % of laboratories using inappropriately high concentrations for non-smokers. They suggest that all clinical laboratories should review their COHb reference range.