Analytical performance of 4 automated assays for measurement of cystatin C.

BACKGROUND: We evaluated the analytical performance of 4 cystatin C assays (Siemens N Latex on BNII, Roche Tina-quant on Cobas c501, Genzyme on Cobas c501, and Tosoh ST AIA-PACK on Tosoh AIA-600II) according to guidelines published by the Clinical and Laboratory Standards Institute. METHODS: We evaluated total imprecision, limit of detection, and limit of quantification for each assay using patient serum pools and linearity/recovery using serial dilutions of a patient serum pool with cystatin C-free serum. We compared patients (n = 102) using the Siemens assay as a comparison method. RESULTS: All assays had limits of detection and quantification <0.08 and <0.39 mg/L, respectively. Total CVs were generally higher than the manufacturers' claims for all assays. The Roche assay overrecovered cystatin C, particularly at low concentrations (mean recovery 119%, 142% at 0.587 mg/L). Deming regression equations were y = 1.184x + 0.089, S(y|x) = 0.246 for Genzyme; y = 0.937x + 0.231, S(y|x) = 0.231 for Roche; and y = 1.010x + 0.216, S(y|x) = 0.115 for Tosoh. The Genzyme assay appeared to report higher results than the Siemens assay, which is consistent with a higher reference interval specified by the manufacturer. CONCLUSIONS: Although all assays were acceptable for clinical use, their diagnostic performances were not optimal. Limitations include imprecision greater than claimed, overrecovery for the Roche assay on low concentration samples, and differences in results for patient samples. The latter situation requires assay-specific cystatin C-based glomerular filtration rate prediction equations at least until calibration is standardized using the international cystatin C calibrator now being developed.

Comparison of the CEDIA and MEIA assays for measurement of tacrolimus in organ transplant recipients.

OBJECTIVES: Tacrolimus is widely used in organ transplantation. We evaluated 2 immunoassays, CEDIA and MEIA, for the measurement of whole blood tacrolimus concentrations. METHODS: For each assay the following were evaluated: total precision, limit of detection (analytical sensitivity), limit of quantitation (functional sensitivity), linearity, and accuracy. Patient correlation studies were performed, comparing each assay with liquid chromatography-tandem mass spectrometry (LC-MS/MS). RESULTS: Total precision for MEIA, corresponding to mean concentrations of 6.8 and 22.5 ng/ml, was 17.4 and 11.9%, respectively. The limit of detection was 0.9 ng/ml, and the limit of quantitation was 4.7 ng/ml. Total precision for CEDIA, corresponding to mean concentrations of 5.3 and 19.9 ng/ml, was 20.6 and 6.3%, respectively. The limit of detection was 0.8 ng/ml, with a limit of quantitation of 4.9 ng/ml. Analysis of proficiency material demonstrated acceptable performance for both assays. In addition, both assays were acceptably linear over their respective reportable ranges. Analysis of patient correlation data using Passing-Bablok analysis and Bland-Altman plots demonstrated a positive average bias for both assays versus LC-MS/MS results. CONCLUSION: Based on our evaluation, both assays demonstrated acceptable performance for use in clinical monitoring of tacrolimus.

Complexed prostate-specific antigen as a staging tool for prostate cancer: a prospective study in 420 men.

Over time, the parameters commonly used to predict pathological stage in men with localized prostate cancer have changed, and there is now little stratification in pretreatment prostate-specific antigen (PSA) concentrations, clinical stages, and biopsy Gleason scores. This prospective study evaluated the utility of complexed PSA (cPSA ) for predicting organ-confined disease in a contemporary series of subjects. The age range of the 420 men was 39 to 72 years (58.2 +/- 6 years). Specimens were collected before radical prostatectomy, and total and free PSA (Hybritech Tandem assays, Beckman Access; Beckman Coulter, Inc., Brea, CA) and total and cPSA (Bayer Immuno 1; Bayer Corporation, Tarrytown, NY) were measured. Pathologic stage was determined from the prostatectomy specimen. Of the 420 men, 316 (75%) had organ-confined disease, and 104 (25%) had non-organ-confined disease (20.7% had extraprostatic extension, 2.6% had seminal vesicle involvement, and 1.4% had positive lymph nodes). Prebiopsy Gleason score distribution was as follows: organ-confined organ-confined, 6 (87%) and 7 (10%); non-organ-confined, 6 (66%) and 7 (30%). Of patients with organ-confined disease, 75% had clinical stage T1c disease compared with 56% for non-organ-confined disease. Using univariate logistic regression, the following variables predicted organ-confined disease: biopsy Gleason score, clinical stage, total PSA, percent free PSA, cPSA, percent cPSA (P <0.05). A multivariate model with biopsy Gleason score, clinical stage, and cPSA had a receiver operator characteristic area under the curve of 0.69. Replacing cPSA with total PSA in this model provided similar information. cPSA and total PSA were highly correlated (r = 0.985). In summary, cPSA was equivalent to total PSA in predicting organ-confined disease. Present and future models and nomograms using PSA as an indicator of pathological stage could consider use of cPSA.

Short-term stability of the molecular forms of prostate-specific antigen and effect on percent complexed prostate-specific antigen and percent free prostate-specific antigen.

Differences in stability of the free and complexed molecular forms of prostate-specific antigen (PSA) may influence the clinical utility of assays for these forms, as well as the calculated ratios to total PSA (tPSA), such as percent free PSA (fPSA) and percent complexed PSA (cPSA). The objective of this study was to directly compare the short-term stability of fPSA and cPSA under different storage conditions. Specimens (3 with prostate cancer, 3 biopsy-negative without cancer, 2 normal) from 8 men were analyzed at baseline within 2 hours of collection, and at 4 hours, 8 hours, 24 hours, 48 hours, and 1 week after storage at room temperature, 4 degrees C, or -20 degrees C. Serum specimens were analyzed in duplicate on the Bayer Immuno 1 analyzer (tPSA, cPSA) and on the Beckman Coulter Access analyzer (tPSA, fPSA Tandem assays). Baseline tPSA values ranged from 0.7 to 62.0 ng/mL, with a median of 7.9 ng/mL (Immuno 1). Overall, all forms of PSA were stable up to 24 hours at the 3 temperatures, with the exception of fPSA and percent fPSA, which decreased when stored at 4 degrees C. After 1 week, tPSA levels decreased when stored at room temperature and at 4 degrees C, as did cPSA stored at room temperature. Over the 7 days, percent cPSA was stable at room temperature, but increased at 4 degrees C. There were no significant changes in any PSA form or calculated ratio with storage at -20 degrees C for up to 1 week. In summary, in the short term (<1 week), fPSA is less stable with storage than tPSA or cPSA in a time- and temperature-dependent fashion. Thus, specimen handling should be considered when interpreting PSA results. It is recommended that specimens not analyzed the same day (within 8 hours of collection) be stored frozen at -20 degrees C.