Interpretation of serum PTH concentrations with different kits in dialysis patients according to the KDIGO guidelines: importance of the reference (normal) values

Background. The recommended target range for serum parathyroid hormone (PTH) in dialysis patients has changed from 150 to 300 pg/mL in the KDOQI guidelines to two to nine times the upper normal limit in the KDIGO ones. Although inclusion/exclusion criteria for the reference population are highly important, they are usually not mentioned in the commercial kits. In this study, we used the same reference population of vitamin D-replete normal subjects to establish reference values for 10 commercial PTH kits. We evaluated whether this may improve the classification of dialysis patients according to the KDIGO compared to the use of reference values proposed by the manufacturers. Methods. We measured serum PTH with 10 different kits in 149 haemodialysis patients, and 240 25-OH-vitamin D-replete (> 75 nmol/L) individuals with an estimated glomerular filtration rate > 60 mL/min/1.73 m(2). Results. For the 10 kits, our upper normal limit was lower than those of the manufacturers. The difference was, however, variable from one kit to another. The two kits that yielded the lowest and the highest absolute concentrations classified differently 84/149 patients (56.4%) according to the KDOQI and 53/149 (36.2%) according to the KDIGO using the manufacturers' normal values. Using our normal values significantly decreased the discrepancies with 24/149 patients (16.1%) being still classified differently. Taking the measurement uncertainty into consideration, 8% of the patients only remained differently classified by these two kits. Conclusions. Using the same vitamin-D-replete population to establish the reference range for 10 commercial PTH kits significantly improved the classification of haemodialysis patients according to the KDIGO target range.