Clinical significance of surfactant protein D as a serum marker for evaluating pulmonary fibrosis in patients with systemic sclerosis

Objective. To determine the clinical significance of surfactant protein D (SP-D), a useful marker for evaluating various lung diseases, in patients with systemic sclerosis (SSc) and to clarify any clinical significance between SP-D and KL-6, which is known to be correlated with pulmonary fibrosis (PF) in SSc patients. Methods. We used a Specific enzyme-linked immunosorbent assay to measure serum SP-D levels in 83 patients with SSc and 31 healthy control subjects. Results. The serum levels of SP-D were significantly higher in patients with SSc than in healthy controls (mean ± SD 81.9 ± 59.2 versus 34.8 ± 13.7 ng/ml). Serum SP-D levels in patients with diffuse cutaneous SSc were significantly higher than those in patients with limited cutaneous SSc (98.8 ± 72.1 versus 66.8 ± 40.0 ng/ml). Serum SP-D levels in patients with PF were significantly elevated compared with those in patients without PF (99.7 ± 64.1 versus 65.3 ± 49.4 ng/ml). Moreover, the incidences of decreased percentage diffusing capacity for carbon monoxide and decreased percentage vital capacity were also significantly greater in patients with elevated SP-D levels than in those with normal levels (67% versus 43% and 36% versus 17%, respectively). There was a significant positive correlation between serum levels of SP-D and KL-6. Serum SP-D and KL.6 levels showed almost the same sensitivities and specificities in the diagnosis of PF (68% versus 73% and 70% versus 74%, respectively). These two markers also predicted PF to almost the same degree (31% versus 33%, respectively). Conclusion. These results suggest that SP-D, as well as KL-6, may be a useful serum marker for evaluating PF in patients with SSc.