Parathyroid hormone measurement in chronic kidney disease: Impact of inter-method variability on mineral bone disease...

Parathyroid hormone (PTH) is measured routinely as part of Chronic Kidney Disease Bone and Mineral Disorders (CKD-MBD) assessment. Multiple PTH assays exist with known differences resulting in CKD-MBD guidelines recommending treatment based on assay-specific thresholds. The study objectives are to assess between manufacturer and within manufacturer variability of PTH assays and the impact of assay variability on the assessment of CKD-BMD using both vendor defined and empirically derived thresholds.

Data were collected from Ontario, Canada's Proficiency Testing Program (24 challenge vials, 115-133 laboratories all using secondary generation PTH assays. Mean PTH and precision by the coefficient of analytical variation (CV_a) were calculated. For each vial, whether the manufacturer's mean value exceeded the vendor-defined and empirically-derived upper limit of normal (ULN) was recorded and the concordance between assays was determined.

Across all laboratories, the mean PTH range was 12.0 ± 3.9 pmol/L and the mean CV_a was 30%. The percent of vials with a mean PTH exceeding manufacturer's specific ULN varied substantially between manufacturers. Only 58% of vials had complete concordance as to whether mean PTH was above assay-specific ULNs. This increased to 83% using the empirically derived ULN.

CKD-BMD assessment and management will depend on the PTH assay. The between-assay variability is reduced but not eliminated when empirically derived reference intervals are used. Improvements in PTH measurement are required in order to ensure consistent patient care.