Hematocrit determination in small bore tubes from optical density measurements under white light illumination

The relationship between the hematocrit of blood flowing in small bore glass tubes and its optical density (OD) under white light (tungsten) illumination has been examined for tube luminal diameters of 40, 50, and 70 μm. The absorption characteristics of hemoglobin solutions under tungsten illumination were also evaluated, resulting in the measurement of an effective extinction coefficient of 1.84 × 10³ cm²/mmole which appeared to be invariant with pO₂, due to averaging over the tungsten spectrum. Upon expressing the measured OD of whole blood as the sum of two components, one representative of light absorption and the other representative of cell-to-cell scattering of light, agreement with the trends described by the multiple scattering theory of V. Twersky (1970, J. Opt. Soc. Amer. 60, 1084–1093), was achieved within 20% (rms error). Within this framework, an explanation is offered for the basis of an empirical correlation between OD and a polynomial function of packed cell fraction (H_T) and tube diameter (D_T), previously described by R. J. Jendrucko and J. S. Lee (1973, Microvasc. Res. 6, 316–333). By choosing a function X = D_T(H_T − H²_T), which appears as an independent variable in the Twersky formulation, a graphical correlation between OD and X can be established in vitro to provide a means of assessing H_T from measurements of OD and D_T. The accuracy of such determinations of H_T is about 26% provided that H_T < 0.50, and red cell velocities are above 1–2 mm/sec. For H_T < 0.20, OD varies strongly with H_T, due to large changes in the scattering term. For 0.20 < H_T < 0.50, OD is a comparatively weak function of tube hematocrit, due to the predominance of a relatively invariant scattering term. As red cell velocities are decreased below 1 mm/sec, the light scattering characteristics change dramatically due to red cell aggregation.