Low plasma-25-hydroxyvitamin D without osteomalacia

Noninvasive testing in the diagnosis of osteomalacia

PURPOSE

Osteomalacia is associated with many clinical, radiographic, and biochemical abnormalities. Unfortunately, none of these are pathognomonic of the disorder, and histologic examination of a bone biopsy specimen is often necessary to confirm the diagnosis. Noninvasive methods of diagnosis would be preferable to decrease patient morbidity and increase cost-effectiveness. Previous studies addressing the adequacy of these methods were performed prior to the widespread availability of 1,25-dihydroxycalcitriol (1,25-(OH)2D3) and parathyroid hormone (PTH) levels. Therefore, we reviewed our experience with patients with biopsy-proven osteomalacia to determine if PTH or 1,25(OH)2D3 levels would serve a useful role in establishing the diagnosis of osteomalacia.

METHODS

We retrospectively studied 17 patients who had biopsy-proven osteomalacia (defined as an osteoid volume greater than 10% and an osteoid width greater than 15 microns) in order to ascertain if their clinical presentation, biochemical profile, and radiographic features were sufficient to establish a diagnosis of osteomalacia.

RESULTS

We found that 94% of our patients exhibited symptoms of osteomalacia, and all patients had at least one demonstrable sign of osteomalacia on examination. Biochemically, the patients presented a heterogeneous picture: 94% had an elevated alkaline phosphatase level; 47% had either a low serum calcium or phosphate (12% had both) level; urinary calcium excretion was low in 18%; and 25-(OH)D3 concentration was low in 29%. Levels of 1,25-(OH)2D3 were measured in eight patients; three had low values despite normal 25-(OH)D3 levels. PTH levels were elevated in 41% of patients in the absence of histologic evidence of hyperparathyroidism; however, these patients were noted to have multiple other abnormal clinical, biochemical, or radiograph features. Radiographically, 18% of patients had pseudofractures; the rest had nonspecific findings suggestive of osteomalacia. In summary, all patients had at least two of the following abnormalities: low calcium, low phosphate, elevated alkaline phosphatase, or a radiographic finding suggestive of osteomalacia.

CONCLUSION

We conclude: (1) a careful history and physical examination remain important in the evaluation of potential osteomalacia patients; (2) PTH offered no apparent benefit as a screening test in our patients; (3) 1,25-(OH)2D3 was low in three patients with normal levels of 25-(OH)D3; (4) decreased urinary calcium excretion had low sensitivity for detecting osteomalacia; and (5) serum calcium, phosphate, alkaline phosphatase, and radiographic examination may be adequate screening tests in patients who have a clinical presentation suggestive of osteomalacia.

Privational and malabsorption metabolic bone disease: plasma vitamin D metabolite concentrations and their relationship to quantitative bone histology

Plasma 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and 25-hydroxyvitamin D (25OHD) concentrations were measured in twenty patients with metabolic bone disease due either to privational causes (10 patients) or malabsorption (10 patients). Abnormally low plasma 1,25(OH)2D3 levels were found in eleven patients, six with privational and five with malabsorption bone disease. Normal plasma 1,25(OH)2D3 concentrations were found in the remaining nine patients; of these, five were either receiving anticonvulsant therapy or had been hospitalised prior to investigation. In the absence of either of these factors, normal plasma 1,25(OH)2D3 levels were only found in patients with malabsorption-associated bone disease. Plasma 25OHD levels were below normal in eleven patients; six had malabsorption and five had privational bone disease. In the fifteen patients not receiving anticonvulsants there were significant inverse correlations between plasma 1,25(OH)2D3 levels and the osteoid volume, surface and seam thickness index. This study indicates that plasma 1,25(OH)2D3 concentrations are low in privational osteomalacia in the absence of anticonvulsant therapy or hospitalisation, although normal levels may occur in malabsorption metabolic bone disease uncomplicated by these factors. The plasma 1,25(OH)2D3 concentration appears to be inversely related to the histological severity of bone disease in patients not receiving anticonvulsant therapy.

Plasma binding capacity for 25-hydroxy-vitamin D in the elderly

Plasma 25-hydroxy-vitamin D (25-OHD) concentrations and 25-OHD binding capacities were measured in 14 elderly patients and in 14 young controls. Both 25-OHD concentrations and 25-OHD binding capacities were reduced in the elderly. In neither group, however, was there a significant correlation between 25-OHD binding capacities and 25-OHD concentrations. Thus, although 25-OHD binding capacities are reduced in sick old people this does not account for the low 25-OHD concentrations often found in this group.