Circulating surfactant protein D is associated to mortality in elderly women: a twin study

Surfactant protein D (SP-D) is produced in the lungs and additional mucosal surfaces. Systemic SP-D levels are previously associated to aging-related- and lifestyle-related disorders and predicts mortality in cardiovascular and lung diseases. However, the association between higher serum SP-D levels and mortality in the general population is unknown. We hypothesized that increased systemic levels of SP-D may be used as prognostic factor for assessing the mortality in the elderly.

SP-D serum levels were measured in 689 elderly subjects and mortality ratios were investigated after a 13-year follow-up period. Survival analysis showed that increasing quartiles of serum SP-D levels were associated to mortality in 70+ year old women (hazard ratio [HR], 1.29; 95% confidence interval [CI], 0.93-1.78; p = 0.032) adjusted for age, smoking and BMI. Women with SP-D levels above 2100 ng/ml had significantly increased mortality when compared to elderly women with SP-D levels equal to or below 2100 ng/ml (HR, 1.45; 95% CI, 1.12-1.88; p = 0.005). The likelihood that the female twin with the highest SP-D level died first increased with increasing SP-D levels (p = 0.031) - that is, the bigger intra-pair difference in SP-D level, the higher the probability that the twin with the highest measure died first (odds ratio [OR], 1.66; p = 0.047).

The study demonstrates that higher circulating SP-D levels are associated with increased mortality rate in elderly women in this population-based cohort study. SP-D may serve as a biomarker to track the cardio-pulmonary health status in elderly women.