Impact of the method of calculation on assessment of the PTH-calcium set point

Background. Although the methodology for calculating the PTH secretory parameters is well established, a consensus on a common methodology for calculation of the set point value has not yet been achieved. This is probably one of the major reasons for the conflicting results obtained for this secretory parameter. The aim of the present study was to analyse the influence of the different methods of calculation on the values of set point obtained in clinical nephrology practice. Methods. We analysed 68 PTH-calcium sigmoidal curves, obtained by infusion of 37 mg/kg Na-2-EDTA i.v. in 2 h and 8 mg/kg Ca gluconate based on the calcium element i.v. in 2 h on two separate days. The set point was calculated according to three different methods: method A, the originally described method, based on the classical four-parameter model, which considers the set point as the calcium concentration corresponding to the PTH value intermediate between the maximal and minimal values (the midrange value method); method B (set point = calcium concentration corresponding to 50% of maximal PTH), and method C (set point=calcium concentration corresponding to 50% inhibition of basal PTH value). The three different sets of set point values were entered into the formula of the sigmoidal curve to test the best fitting of the PTH experimentally observed values. Results. The set point values calculated with the classical midrange value method were lower than the corresponding values calculated by the other two methods; method C gave the highest values. Furthermore the best fitting of the experimentally observed PTH levels was obtained by method A, the worst by method C, while method B gave intermediate results. The difference between method A and method B was analysed in order to see if this difference is constant over the whole range of PTH secretory conditions and calcium concentrations. The higher the basal serum calcium concentrations and the lower the suppressibility of PTH, the greater was the overestimation of set point values by method B compared to the midrange value method. Conclusions. Method A, the midrange value method, gives the set point values closest to the original concept of the four parameter model. Although method B (50% of maximal PTH) is well correlated with the original method, it overestimates the set point values and most importantly, this overestimation is not constant, but largely affected by calcium concentration and by the secretory conditions of parathyroid glands.