Table 2.

Summary of cystine assay performance characteristics.

Assay parameterAssay characteristics
Assay formatSpectrophotometry
Analyte, unitCystine, mg/L
Assay time30 min
StandardsHigh: cystine 100 mg/L (prepared in 10 mmol ammonium bicarbonate)
Low: blank (CLRW)
Sample typeUrine, pH >8
Amount of sample required50 μL
Total assay volume190 μL
Number of samples per assay20
LOB = meanblank + 2 SD2.7 mg/L, n = 40
LOD = LOB + 2 SDlow [analyte]11.9 mg/L, n = 40
LOQ: lowest [analyte] with CV <20%15.3 mg/L, n = 39, SD 2.2 mg/L, CV 14.6%
AMR: LOQ and high standard15.3 mg/L and 100 mg/L
Intraassay variation (2 levels, 5 replicates each day for 5 days)High: mean 83.6 mg/L, SD 8.0 mg/L, CV 9.6%
Low: mean 36.4 mg/L, SD 6.6 mg/L, CV 18.1%
Interassay variation (2 levels, 5 replicates each day for 5 days)High: mean 83.6 mg/L, SD 6.7 mg/L, CV 8.0%
Low: mean 36.4 mg/L, SD 6.4 mg/L, CV 17.6%
Stability (n = 7) (7-day acceptance criteria: <3 samples with >20% difference, mean % difference <20%)Ambient (20 °C), 1 sample >20%, mean 13.4%