Comparison Study of Urinary Pyridinoline and Deoxypyridinoline Measurements in 13 US Laboratories

Abstract

Measurement of biochemical bone markers is commonly used in the management of various metabolic bone diseases (1)(2). The pyridinium crosslinks pyridinoline (PYD) and deoxypyridinoline (DPD) are well-characterized markers for bone resorption that have been available for several years (3). Assays to measure the sum of free and peptide-bound urinary PYD or DPD (total PYD or DPD) or free, non-peptide-bound molecules have been developed and described (4). Analytical variability of PYD and DPD measurement is a major problem hampering comparability and interpretation of results. As part of the CDC program to develop a reference system to standardize the measurements of PYD and DPD, we conducted a round-robin interlaboratory comparison study to assess the state of analytical variability. We invited laboratorians within the US involved in routine measurement of urinary DPD and/or PYD to participate in this study. Participants were asked to analyze five identical double-blinded sets of six unknown samples for PYD, DPD, and creatinine on 5 days in duplicate. Information was collected from each participant on sample handling and preparation, as well as calibrators, including information on sample hydrolysis. Urine was collected in agreement with CDC Institutional Review Board regulations. We screened individual urine samples for total PYD and DPD concentrations using our in-house HPLC assay (5)(6) and then combined them into three concentrations, from normal to moderately increased pyridinium crosslink concentrations (low-, medium-, and high-pool). We created a fourth urine pool by mixing the low and medium pools (1:1 by volume; mixed pool). One part of the mixed pool was used for the addition of PYD and DPD calibrators (supplemented pool; 638 nmol/L PYD and 219 nmol/L DPD). An aqueous solution of DPD and PYD calibrators (aqueous sample; 306 nmol/L PYD and 555 nmol/L DPD) was included as the sixth sample. We tested immunoreactive, …