Long-term stability and circadian variation in circulating levels of surfactant protein D

Surfactant protein D (SP-D) is an oligomeric calcium-dependent lectin with important roles in innate host defence against infectious microorganisms. Several studies have shown that patients with inflammatory lung disease have elevated levels of circulating SP-D, and serum SP-D has been suggested to be used as a biomarker for disease e.g. in COPD. We aimed to investigate the variation of circulating SP-D in healthy individuals in and between days for 6 months. In addition, we studied the SP-D response to a standardized physical exercise programme. SP-D was measured in serum using a 5-layered ELISA technique. We found that circulating SP-D remained constant over a 6-month period. However, during the course of one day SP-D varied significantly. Median SP-D peaked at 10 a.m. at 1009 ng/ml (95% CI: 803-1497), subsequently decreasing to nadir at 10 p.m. at 867 ng/ml (95% CI: 650-1148)(P<0.00005). Median pre-exercise level of SP-D was 746 ng/ml (95% CI: 384-2035), and immediately after cessation of physical activity the median SP-D level was 767 ng/ml (95% CI: 367-1885) (P=0.248). Our findings underscore the importance of standardized blood sampling conditions in future studies on the potential role of SP-D as a biomarker. Importantly, stable measures of systemic SP-D over a prolonged period support that SP-D is suitable for biomarker studies.