MLN4924 Finally, we calculated the diagnostic sensitivity and specificity for several cutoff concentration values of CSF cystatin C. Total cystatin C concentration measurements were found to have better diagnostic parameters than percent cystatin C values. A cutoff value of 2.20 mg/ml identified a small subset of ALS patients with relatively high specificity. A cutoff value of 2.70 mg/ml identified a modest subset of ALS patients while maintaining high specificity versus controls. A less conservative cutoff value of 3.50 mg/ml identified a majority of ALS patients, but demonstrated lower specificity. As noted above, cystatin C was previously reported to be significantly reduced in the CSF of ALS patients using mass spectrometry-based proteomics, but the between-group differences based on our ELISA data were less robust. To explore the relationship between CSF cystatin C levels measured by these two techniques, we compared our ELISA results with SELDI-TOFMS data for the same CSF samples. We found significant, positive correlations between the 13.3 kDa SELDI-TOF-MS mass peak intensity for cystatin C and both total cystatin C and percent cystatin C protein levels as measured by ELISA. However, the correlation coefficients suggest that these techniques may be differentially sensitive to various modified forms of native cystatin C. We repeated the group analysis for the diagnostic utility of cystatin C in plasma, and both measures of cystatin C varied significantly by diagnosis and age, but not by gender. Post-hoc analyses revealed that total and percent cystatin C were significantly increased in both ALS patients and Y-27632 disease controls relative to healthy controls. However, there were no differences in cystatin C levels between ALS patients and disease controls. Identical trends were observed for all subgroup analyses of cystatin C levels in plasma. To further characterize the relationship between CSF and plasma cystatin C levels, we assessed the correlation between CSF and plasma cystatin C levels for individual subjects. To be clinically useful as a diagnostic biomarker, cystatin C must also be able to differentiate between ALS patients and individuals with neurologic diseases that closely resemble ALS, or ALS ����mimic diseases.���� A recent study reported a significant reduction in CSF cystatin C levels in ALS patients relative to polyneuropathy patients. In our ELISA analysis, cystatin C was reduced in the CSF of ALS patients relative to all DC combined and, to a greater degree, relative to a mimic disease control group that included a variety of ALS mimics, but neither difference reached statistical significance.