[Validity of serum surfactant protein D measurements in a cross-sectional study]

In October, 2008, the method for enzyme-linked immunoassay for serum surfactant protein D (SP-D) was changed from SP-D kit (Yamasa) EIA to SP-D kit (Yamasa) EIA2 in Japan. In our follow-up survey on 144 indium-exposed workers from December, 2008 to June, 2009, SP-D showed systematically higher values compared to the first survey from March to July, 2008. Geometric means (prevalence rate) of serum SP-D of the first and follow-up study were 52.1 ng/mL (4.1%) and 72.1 ng/mL (14.6%), respectively, though serum KL-6 and SP-A were stable. Thus, we examined the validity of the old and new methods.

As a result of examining about 200 subjects, the correlation between the two methods was good. However, as a reproducibility examination of about another 300 subjects between the post-measured and the pre-measured values with the same new method at Laboratory A, the differences between vales in each observation are not acceptable. Especially, examinations performed using the lot 804, 805, and 105 at laboratory A showed systematic higher values than others.

Various factors might have caused the large measurement bias; in particular, the new method lacks sufficient reproducibility between lots. When observing the chronological change of SP-D, attention must be given in comparisons of serum SP-D after October, 2008 to before that because of the metrology-dependent measurement bias.