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We developed a novel immunoradiometric assay (IRMA; whole parathyroid hormone [PTH] IRMA) for PTH, which specifically measures biologically active whole PTH(1-84). The assay is based on a solid phase coated with anti-PTH(39-84) antibody, a tracer of 125 I-labeled antibody with a unique specificity to the first N-terminal amino acid of PTH(1-84), and calibrators of diluted synthetic PTH(1-84). In contrast to the Nichols intact PTH IRMA, this new assay does not detect PTH(7-84) fragments and only detects one immunoreactive peak in chromatographically fractionated patient samples. The assay was shown to have an analytical sensitivity of 1.0 pg/ml with a linear measurement range up to 2300 pg/ml. With this assay, we further identified that the previously described non-(1-84)PTH fragments are aminoterminally truncated with similar hydrophobicity as PTH(7-84), and these PTH fragments are present not only in patients with secondary hyperparathyroidism (2°-HPT) of uremia, but also in patients with primary hyperparathyroidism (1°-HPT) and normal persons. The plasma normal range of the whole PTH(1-84) was 7-36 pg/ml (mean ؎ SD: 22.7 ؎ 7.2 pg/ml, n ‫؍‬ 135), whereas over 93.9% (155/165) of patients with 1°-HPT had whole PTH(1-84) values above the normal cut-off. The percentage of biologically active whole PTH(1-84) (pB%) in the pool of total immunoreactive "intact" PTH is higher in the normal population (median: 67.3%; SD: 15.8%; n ‫؍‬ 56) than in uremic patients (median:53.8%; SD: 15.5%; n ‫؍‬ 318; p < 0.001), although the whole PTH(1-84) values from uremic patients displayed a more significant heterogeneous distribution when compared with that of 1°-HPT patients and normals. Moreover, the pB% displayed a nearly Gaussian distribution pattern from 20% to over 90% in patients with either 1°-HPT or uremia. The specificity of this newly developed whole PTH(1-84) IRMA is the assurance, for the first time, of being able to measure only the biologically active whole PTH(1-84) without cross-reaction to the high concentrations of the aminoterminally truncated PTH fragments found in both normal subjects and patients. Because of the significant variations of pB% in patients, it is necessary to use the whole PTH assay to determine biologically active PTH levels clinically and, thus, to avoid overesti-

Abstract:Objective: Knee osteoarthritis is a highly prevalent disease, commonly requiring joint replacement; it substantially reduces quality of life and increases health care utilization and costs. We aimed to identify whether quantitative measures of articular cartilage structure predict knee replacement, and to establish their utility as outcomes in clinical trials of disease modifying therapy. Methods:A nested case-control study was performed in Osteoarthritis Initiative participants, a multicenter observational cohort of 4796 participants with or at risk of knee osteoarthritis. 127 knees were replaced between baseline and four years follow-up, and one control knee per case matched for baseline radiographic disease stage (KellgrenLawrence grade, KLG), gender, and age. Quantitative cartilage measures were obtained from 3 Tesla magnetic resonance images at the exam before knee replacement, and longitudinal change during the prior 12 months when available (n=110).Results: Cartilage thickness loss in the central and total medial femoro-tibial compartment (primary and secondary outcomes) was significantly greater in case than in control knees (AUC=0.59/ 0.58). Differences in cartilage loss were greater at earlier than at later radiographic disease stages (p<0.01 for interaction with KLG). Cartilage thickness loss in the central tibia was the most predictive longitudinal measure (AUC=0.64). Cross-sectionally, denuded bone areas in the medial femur distinguished case and control knees most strongly (AUC=0.66). Conclusions:This study demonstrates the predictive value of quantitative, magnetic resonance imaging-based measures of cartilage for the clinically relevant endpoint of 4 knee replacement, providing support for their utility in clinical trials to evaluate the effectiveness of structure modifying intervention.

In 8 adolescents with end-stage renal disease (ESRD), basal PTH concentrations measured with a novel immunoradiometric assay (IRMA) (Scantibodies Laboratory, Inc.; S-IRMA) were invariably lower than those estimated with an established assay (Nichols Institute; N-IRMA) (263 +/- 228 versus 645 +/- 442 pg/ml, respectively; p<0.00001). During in vivo dynamic testing, set points for calcium-regulated PTH release were indistinguishable for both IRMAs (1.21 +/- 0.05 versus 1.22 +/- 0.06). However, maximal PTH concentrations were significantly lower when measured by S-IRMA then by N-IRMA (557 +/- 448 and 1114 +/- 606 pg/ml, respectively); minimum PTH concentrations were 41 +/- 65 pg/ml (5.0 +/- 4.2% of maximum) and 189 +/- 137 pg/ml (13.6 +/- 7.2% of maximum), respectively. Correlation between PTH and blood ionized calcium indicated that PTH measured by S-IRMA decreased more readily than the concentrations determined by N-IRMA. The N-IRMA showed indistinguishable cross-reactivity with hPTH(1-84) and hPTH(7-84), while the S-IRMA detected only the full-length peptide. Furthermore, the radiolabeled detection antibody of the N-IRMA interacted equivalently with hPTH(1-34) and hPTH(2-34), while the S-IRMA showed crossreactivity only with hPTH(1-34). These differences in assay specificity could explain the observed differences in ESRD, and suggest that PTH concentrations estimated by the S-IRMA reflect more accurately the amount of biologically active PTH in the circulation. Since low concentrations of PTH are frequently associated with adynamic bone disease, our findings may have significant implications for the treatment of renal osteodystrophy with calcium and/or biologically active vitamin D analogs.