Parathormone Levels in a Middle-Eastern Healthy Population Using 2^nd and 3^rd Generation PTH Assays

The purpose of the current study is to determine PTH reference values in vitamin-D-replete Lebanese adults using 2^nd and 3^rd generation PTH assays and to look at the factors that affect PTH variations.

Fasting PTH was measured using 2^nd and 3^rd generation Diasorin PTH assays in 339 vitamin-D-replete healthy subjects aged 18 to 63 years (230 men and 109 women) who have normal calcium levels and an eGFR ≥60 ml/mn. 25-OH vitamin D (25(OH)D) was measured using the Diasorin assay.

For the 2^nd PTH generation, median (IQR) levels were 48.9 (34.9-66.0) pg/ml, and its 2.5^th-97.5^th percentile values were 19.7-110.5 pg/ml for 25(OH)D values between 20 and 30 ng/ml, and 19.7-110.7 pg/ml for 25(OH)D values ≥30 ng/ml. For the 3^rd PTH generation, the median (IQR) values were 23.9 (17.7-30.5) pg/ml, and its 2.5^th-97.5^th percentile values were, respectively, 9.2 and 50.2 pg/ml for 25(OH)D values between 20 and 30 ng/ml, and 8.4 and 45.4 pg/ml for 25(OH)D values ≥30 ng/ml. The median (IQR) serum 25(OH)D levels were 27.5 (23.8-32.7) ng/ml. 2^nd and 3^rd generation PTH values are strongly correlated (r = 0.96, p < 0.0001), but poorly concordant (Lin's concordance coefficient 0.365, 95% CI: 0.328-0.401) with observations beyond the 95% Bland-Altman limits of agreement. 2^nd and 3^rd generation PTH levels did not differ according to gender and were significantly correlated with age but not with 25(OH)D and serum calcium levels.

Lebanese adult healthy subjects have higher 2^nd and 3^rd generation PTH levels compared with the reference range provided by the manufacturer. The reference range was not influenced by changing the 25(OH)D cutoff. The clinical significance of the higher PTH levels in our population should be investigated.