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.