Biological variation of soluble CD163

A soluble plasma form of CD163 (sCD163) was recently identified. The protein has anti-inflammatory effects in vitro and is elevated in patients with myelo-monocytic leukaemia and infection. For rational use and evaluation of this potential new quantity it is important to have knowledge of its biological variability and to use a methodology that has a sufficiently analytical quality. The day-to-day and diurnal biological variabilities of sCD163 were studied in 12 healthy people using a sandwich ELISA. The within-run-, between run- and total analytical coefficients of variation were estimated to 3.6%, 4.8% and 6.0%, respectively. The day-to-day within-subject biological variation was estimated to 9.0%, and the between-subject biological variation to 35.9%. A diurnal variation in sCD163 concentrations with 14% lower values in the night (supine position) was observed. The ratio between total analytical variation and within-subject biological variation was 0.67. The index of individuality, calculated as the ratio between within-subject biological variation and between-subject biological variation, was low and similar to complement factors and immunoglobulins. A low index of individuality is important in a monitoring situation where small changes from the set point of the individual can be detected in serial measurements. For this purpose, the critical difference for a series of results in the same patient (significant at p < 0.05) was calculated to 30% for samples taken on different days and measured in separate runs.