Variability and Response of Urinary Resorption Markers to Hormone Replacement Therapy (1999)

To the Editor: Hannon et al.1 recently reported a comparison of the performances of a panel of biochemical markers of bone turnover in terms of intraindividual variability (CVi) and response to hormone replacement therapy (HRT). CVi was assessed from four samples collected over 6 months in 11 women without any treatment (mean age 54 years) and response to HRT was investigated in 11 women of similar age treated by transdermal estradiol for 6 months. The reported CVi were similar for the different serum bone formation markers, but large differences were observed for the urinary resorption markers. Surprisingly, CVi was much higher for urinary cross-linked C-telopeptides (CTx) than for cross-linked N-telopeptides (NTx) (47.7% vs. 24.3%). Thus, despite the fact that urinary CTx demonstrated the largest decrease after 6 months of HRT, the number of individuals identified as responders, i.e., patients characterized by a decrease of bone marker level greater than the least significant change (LSC) cut-off, which is based on CVi in controls was the lowest level (2 out of 10). As Hannon et al.,1 we believe that the issue of intraindividual variability of biochemical measurements is of crucial importance to judge the utility of bone markers in monitoring treatment efficacy in a single patient. Following are our results, which differ markedly from those of Hannon et al.1 We measured urinary CTx (Crosslaps ELISA; CIS BioInternational, Saclay, France) on four consecutive urine samples (first morning void and 24-h urine sample) collected every year over 3 years in 259 healthy untreated postmenopausal women aged 51–89 years (mean, 64 years), 1–38 years postmenopause (mean 15 years) who were part of a large prospective cohort (OFELY study). None of the women had bone diseases or treatments that could interfere with bone metabolism (including HRT) during the 3-year period. As shown in Table 1, for the whole population CVi ranged from 24–27% according to the type of urine sample. When CVi was calculated only from the 49 women within 5 years of menopause (mean age 55 years)—a population of similar age as that used by Hannon et al.1—we found very similar figures with a 21% CVi for the first morning void sample. This long-term CVi compares well with the 20–25% reported previously for urinary NTx over periods ranging from 6–15 months.1-3 To have an estimation of the short-term variability, we also measured urinary CTx and urinary NTx on first morning void samples collected each day during 5 days in eight healthy adults (five women, three men). We found similar results for NTx (17%) and CTx (21%), although these figures may—as those reported by Hannon et al.—not be accurate because determined on a small group. Thus, we believe that the CVi assessed on the 259 women over 3 years would represent a reliable estimation of urinary CTx within patient variability. In addition in the study of Hannon et al.,1 urine was collected at an undefined time during the day, which may further increase variability due to the well known diurnal variation of bone resorption markers. Using our estimation of CVi for urinary CTx, the least significance change cut-off where CVa is the analytical CV of the assay, i.e., 7% in our laboratory) was estimated to 69%, i.e., similar to the 70% reported for urinary NTx by Hannon et al.1 We also assessed the response of urinary CTx to HRT in 28 women aged 46–53 years, menopaused from 6 months to 3 years receiving a cyclic combination of estradiol valerate and cyproterone acetate. They followed a 28-day cycle: days 1–11, 2 mg of estradiol valerate; days 11–21, 2 mg of estradiol valerate + 1 mg CPA; days 22–28, no tablet. Two-hour fasting morning urine samples were obtained in all women prior to the study and after 6 months of treatment for measurements of urinary CTx. As shown on Fig. 1, when the change of urinary CTx was calculated as the difference between the level of the marker at 6 months and the level at baseline, expressed as percentage of the baseline level (Fig. 1A), we found a mean decrease of 59%, which was similar to that reported by Hannon et al.1 in 10 women treated by transdermal HRT for the same duration. Twelve patients out of the 28 (43%) demonstrated a decrease of urinary CTx greatest than the LSC. When the decrease of urinary CTx was expressed as a percentage of the mean of urinary CTx levels at baseline and at 6 months, i.e., the definition used by Hannon et al.,1 the percentage of responders increased to 68%. This rate of responders is similar to the 64%, and 73% reported by Hannon et al.1 for urinary total and free deoxypyridinoline, respectively, and higher than the 36% and 27% observed for free urinary pyridinoline and NTx, respectively. Interestingly, in the study of Hannon et al.,1 adopting the more realistic 69% cut-off as a LSC for urinary CTx, the authors would have detected 8 responders out of 10 patients on HRT. In conclusion, provided that the within patient variability of bone markers is accurately estimated, i.e., on large and representative populations, measurement of biochemical markers appears to be useful to monitor rapidly HRT efficacy in individual postmenopausal women. Individual response of urinary CTx to 6 months of treatment with HRT. Points represent the individual response, defined as the change in marker from baseline to 6 months, expressed as a percentage of the baseline level (A) or as a percentage of the mean of the levels at baseline and at 6 months (B). The dotted horizontal line represents the mean percentage change from baseline. The plain horizontal line is the least significant change (LSC) cut-off. The boxes include patients demonstrating a decrease of urinary CTx at 6 months larger than the LSC, i.e., responders.