Physical properties of sputum. 3. Rheologic variability and intrinsic relationships.

Abstract

The natural variation in viscosity and elasticity in a single 0.5-ml sample of sputum, as well as among several expectorates of a patient during a single day and during a three-day period, has been quantitated. Other properties of sputum, such as gross appearance, cellular density, percentage of solid content, types of cells present, and pH were also studied, and possible correlations were investigated. The viscosity variation (at a shear rate of 1 sec−1) averaged 11 ± 8 per cent (mean ± SD) for a single sample; 26 ± 18 per cent for three expectorates of a single patient during one day; 49 ± 33 per cent for one patient during three days. Variation in elasticity averaged 6 per cent for one sample; 25 ± 15 per cent for one day; 33 ± 9 per cent for three days. Samples classified by eye as mucoid or purulent according to pre-established color and opacity criteria showed a close correlation between this classification and the variables described above. Samples called purulent had a high density of cells with 85 per cent of the samples filling two-thirds or more of a low power (40×) field on wet mount whereas mucoid samples all filled one half or less of the same field. All samples called purulent had a cellular distribution showing more than 70 per cent neutrophils, whereas 80 per cent of mucoid samples had a neutrophil percentage of less than 60 per cent. The concentration of solids for all purulent samples was between 6 and 10 per cent whereas 83 per cent of mucoid samples were between 2 and 5 per cent. Purulent samples had a slightly higher average pH of 7.83 than the 7.59 of mucoid samples. The elastic recoil of purulent samples, measured at 100 dyne per cm2, averaged 4.41 units, which was less than the 6.98 units for mucoid samples. The large variability in viscosity among patients precluded the definition of quantitative differences between purulent and mucoid sputum. Nevertheless, whenever there was a change in the type of sputum of a given patient, purulent sputum was always more viscous.