Suitability of the measurement imprecision of commun urinary biochemical analytes

Background: Imprecision monitoring is central in the performance evaluation of an analytical system. To be suitable in clinical setting, the measurement imprecision should fulfil goals derived from biological variation data of the corresponding analyte, when available. Here we aimed to evaluate the long-term imprecision in our laboratory of measurements of 11 urinary analytes measured on the Roche Cobas 6000 system. Methods: Data were obtained for evaluated analytes by drawing up an internal quality control (QC) programme based on daily measurements of a lyophilized control material (Assayed Urine Control – Level 3, Randox Laboratories, lot no. 580UC). CVs were calculated along the whole examined period (February 2012 to September 2012) (n=165) and compared with corresponding goals for imprecision derived from biological variability data (analyte concentration in 24 h urine sample), with the exception of urinary chloride and glucose for which biological variability data are missing. Results: Analyte concentration means and CVs (optimum goals in parentheses) were as follows: albumin, 182 mg/L, 3.0% (15.3%); calcium, 194 mg/L, 2.9% (6.9%); chloride, 258 mmol/L, 2.2% (not available); creatinine, 1.91 g/L, 2.1% (6.0%); glucose, 2.78 g/L, 1.9% (not available); magnesium, 323 mg/L, 3.5% (11.4%); phosphate, 852 mg/L, 2.0% (6.6%); potassium, 105 mmol/L, 2.1% (6.8%); sodium, 193 mmol/L, 1.9% (6.0%); urea, 14.7 g/L, 2.5% (5.7%); and urate, 214 mg/L, 2.7% (6.2%). Conclusions: Our study shows that in routine laboratory practice and over a clinically and analytically relevant timespan, the imprecision of the common urinary analyte measurements on the Cobas 6000 system fulfils optimum goals of analytical performance. Although intra-individual biological variability of urinary analytes is usually relatively high, resulting in less stringent analytical goals when compared to those of serum measurements, the excellent CVs of urinary analyte measurements qualify the suitability for clinical application of these tests.