We read with interest the article by VanDerMeid et al.
1 The authors found that tear proteins such as MMP-9 correlate with Schirmer test values. However, they did not mention whether the correlation remains after adjustment for the effect of age, a determination that is critical for the interpretation of these data. An analysis of the authors' data in their Table 1 shows that the age of the participants correlated inversely with the Schirmer values in the left eye (
r = −0.415;
P = 0.025), but not in the right.
It has been reported that age correlates with MMP-9 concentration (Markoulli M, et al. IOVS 2010;51:ARVO E-Abstract 4186) in tears collected before sleep (r = 0.46; P = 0.022). This finding suggests that the association reported between MMPs such as MMP-9 and the Schirmer test results is confounded by age.
Supported by National Medical Research Council Singapore Grants NMRC/1206/2009, NMRC/CSA/013/2009, NMRC/TCR/002-SERI/2008, and NMRC/CG/SERI/2010.