The present study was designed to determine the criterion for 1,25-dihydroxyvitamin D, (1,25 (OH),D,) loading test in normal subjects and haemodialysis patients. Fourteen normal subjects were administered 1.0 p,g of 1,25(OH),D, per 0s and serum 1,25(OH),D was monitored every hour up to 6 h afterwards under conditions of overnight fasting, and six haemodialysis patients were administered 2.0 p,g of 1,25(OH),D, per 0s and serum 1,25(OH),D was monitored every 2 h up to 12 h afterwards. Peak time of serum 1,25(OH),D varied between 2 and 5 h after administration in normal subjects. However, there was a good correlation between the maximum increment of 1,25(OH),D (maxA1,25(OH),D) and the increment at 4 h after administration (A1,25(OH),D(4 h)). The peak time of A1,25(OH),D in six haemodialysis patients was also at 4 h after administration. From these observations, A1,25(OH),D(4 h) was evaluated in subsequent studies. Twenty-six normal subjects and 24 haemodialysis patients were administered 0.5-2.0 p,g of 1,25(OH),D, per os, according to their bodyweights, under conditions of overnight fasting. Blood samples were drawn for measuring 1,25(OH),D prior to and 4 h after administration. A1,25(OH),D(4 h) showed good correlation with the dose of 1,25 (OH),D, adjusted by bodyweight (ngkg bodyweight). The ratio of A1,25(OH),D(4 h) and adjusted dose of 1,25(OH),D, was more than 2.0 in all normal subjects (range: 1.97-2.89, mean k SD: 2.38 * 0.287). Moreover, the ratio of A1,25(OH),D(4 h) and adjusted dose of 1,25(OH),D, showed a good reproducibility (CV% = 5.7 f 0.32, n= 5), and did not depend on the administered dose of 1,25(OH),D,, suggesting that this ratio is a good parameter for the intestinal absorption of 1,25(OH),D,.In haemodialysis patients, the mean ratio of A1,25 (OH),D(4 h) and adjusted dose of 1,25(OH)iD, was 2.14 f 0.489, which was not significantly different from the ratio in normal subjects, suggesting that, fundamentally, there was no impairment of intestinal absorption of 1,25(OH),D, in these patients. However, low ratios of A(4 h) and the dose of 1,25(OH),D, with low basal levels of 1,25(OH),D were observed in some patients (less than 1.5 in four patients), suggesting that there exist haemodialysis patients with malabsorption of 1,25(OH),D,. From these results, the criterion for normal response in 1,25(OH),D loading test was proposed, namely, that the ratio of A1,25(OH),D(4 h) and adjusted dose of 1,25(OH),D, be more than 2.0.
