Mineral and bone disease - CKD 5D

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Introduction and Aims: In Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD) patients, most treatment decisions concerning vitamin D use, administration of calcimimetics and parathyroidectomy are guided by parathyroid hormone (PTH) levels. The quality of the PTH test has been judged of paramount importance. Various fully automated assays are currently available, measuring either full-length PTH(1-84) together with N-terminal truncated fragments (immunometric “second generation” tests, misleadingly called “intact PTH assays”) or PTH(1-84) alone (“third generation tests”, so called “biointact PTH assays”). In the present study on hemodialysis patients, we assessed the technical and clinical performance of two novel automated biointact PTH(1-84) assays: the Elecsys® PTH(1-84) assay from Roche Diagnostics (Ro) and DiaSorin's LIAISON® PTH(1-84) assay (DS). Specifically, we aimed to evaluate 1) the comparability of Ro and DS PTH(1-84) levels; 2) the correlation between biointact PTH and intact PTH; 3) the association of PTH values from biointact and intact PTH assays with serum phosphate, as well as 4) with serum creatinine concentrations. Methods: We recorded demographics, patient and dialysis treatment characteristics, and pharmacotherapy for CKD-MBD. Laboratory work-up included total serum calcium, phosphate and creatinine, besides intact Elecsys® PTH (Ro) and biointact PTH(1-84) (Ro and DS). PTH levels were measured on immunoassay analyzers (Ro Cobas e411R and DS LIAISON® ) with the respective test kits. Statistical methods included Passing & Bablok regression fit (aims 1 and 2), Bland-Altmann plots (aim 1), multiple linear regression analysis (aim 3), and linear regression fit (aim 4). Local ethics approval: EK#452/2010. Results: 121 patients on dialysis for 3.5 ± 3.8 years, with serum phosphate 1.9 ± 0.6 mmol/L participated in the study. Intact PTH levels were 374 ± 428 pg/ml, and biointact PTH(1-84) concentrations 231 ± 255 pg/ml (Ro), respectively 235 ± 272 pg/ml (DS; all values on average ± SD). The regression equation Ro = 0.87×DS + 19.60 describes the correlation between both biointact assays (r = 0.98). Bland-Altmann plots revealed an average ± 2 SD bias of 10 ± 27 below 200 pg/ml; above 200 pg/mL the bias was -32 ± 157 (Ro minus DS PTH[1-84]) . Comparisons between biointact PTH(1-84) versus intact PTH are displayed in the figure. The association between PTH and phosphate, with various levels of adjustment, was very similar, regardless of the PTH assay, as shown by only negligible differences in p-values or estimated coefficients of determination for PTH. PTH levels measured with all three assays were significantly associated with serum creatinine, but only in patients with residual renal function ( > 0 mL urine per day; r = 0.44 for both PTH(1-84) assays, r = 0.46 for intact PTH, all p < 0.001). Conclusions: The results obtained by the automated biointact PTH(1-84) assays from Ro and DS were well correlated, but showed increased deviations at higher concentrations. The concentration of PTH(1-84) is roughly two third of the PTH concentration obtained with an intact PTH assay (full-length PTH plus fragments). Importantly, this has to be considered, if published threshold levels obtained by intact PTH assays are used for guideline-based treatment decisions. The association between serum creatinine and PTH levels in hemodialysis patients with residual renal function deserves further study, e.g. investigation of its usefulness as a prognostic marker.

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