Estimation of red blood cell lifespan from alveolar carbon monoxide measurements

Measurement of alveolar carbon monoxide (CO) presents a facile technique to estimate the lifespan, L, of red blood cells (RBCs) in vivo. Several recent studies employ this technique and calculate L (in days) using the expression, L=13.8 [Hb]/P(CO)(end), where [Hb] is the concentration (in g/dL) of hemoglobin in blood, and P(CO)(end) is the endogenous production of CO (in ppm). Implicit in this calculation is the assumption that the fraction, f, of endogenous CO production due to RBC turnover is a constant equal to 0.7, which yields the expected RBC lifespan, L approximately 120 days, in normal controls. In anemic patients, however, enhanced RBC turnover may increase f substantially above 0.7. The above expression then overestimates L. Here, we derive an alternative expression, L=3390[Hb]/322P(CO)(end)-110, that accounts explicitly for the dependence of f on the rate of RBC turnover and thereby provides more accurate estimates of L without requiring additional measurements. Using the latter expression, we recalculate L from recent measurements on hepatitis C virus infected patients undergoing treatment with ribavirin. We find that our estimates of L in these patients (39+/-13 days) are significantly lower than current estimates (46+/-14 days), indicating that ribavirin affects RBC survival more severely than expected from current studies. Our expression for L is simple to employ in a clinical setting and would render the broadly applicable technique of alveolar CO measurement for the estimation of RBC lifespan more accurate.