Laboratory Tests for Assessing Metabolic Bone Disease
A. Calcium is present in serum in three forms. About 50% is bound to proteins, predominantly albumin. About 10% is complexed with anions, such as bicarbonate, phosphate, citrate, etc., and about 40% is in the free, ionized form. The free, ionized form is the physiologically important fraction.

Total calcium is routinely measured in serum or urine specimens by a colorimetric method. The absorption of a dye (most commonly cresolphthalein complexone) intensifies upon interacting with calcium from acidified specimens in which calcium is liberated from bound forms.
Determination of free calcium is not always readily available, but can be measured by ion specific electrode. Total serum calcium concentration is generally proportional to free calcium concentration, so long as albumin concentration is normal. When albumin concentration is abnormal, total serum calcium concentration is interpreted on the basis of what the total calcium concentration would be if albumin concentration were normal by adding 0.8 mg/dl to the calcium value for each 1 g/dl that albumin is less than 4 g/dl, i.e.,

corrected total calcium = measured value + 0.8x(4 g/dl - albumin)mg/dl.

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B. Inorganic Phosphate is routinely measured in serum or urine specimens by a colorimetric method.
C. The Alkaline Phosphatase isoenzyme from bone becomes elevated as a consequence of osteoblast stimulation. Total serum alkaline phosphatase elevations in metabolic bone disease are most often only mild, except in Paget's disease of bone in which cases values are several tens to hundreds times the upper limit of normal.
D. Parathyroid Hormone is measured by radioimmunoassay. Until recently only a few commercial, specialty laboratories provided reliable results. Interpretation of results is complicated by the fact that the intact, circulating hormone is rapidly cleaved by a proteolytic enzyme into N and C terminal fragments. The circulating half life of the N terminal fragment is less than an hour. The C terminal fragment has a longer circulating life time of several hours. Assays are available for the intact hormone, for the N terminal fragment and for the C terminal fragment. Values for the intact hormone are difficult to interpret because the N and C terminal fragments are also included in the measurement but with different cross reactivities and is therefor not clinically useful.
Results from the C terminal assay are useful for deriving information about the functional status of the parathyroid gland. PTH results must be interpreted along with serum calcium results. Elevated PTH values are found in both primary and secondary hyperparathyroidism. PTH is appropriately elevated in secondary hyperparathyroidism in which cases serum calcium values are low or low-normal. PTH is inappropriately elevated in primary hyperparathyroidism in which cases serum calcium concentrations are elevated. Depressed PTH results are found in both primary hypoparathyroidism and in cases of hypercalcemia from excess calcium intake and from osteolytic bone destruction. The N terminal assay is used when blood specimens are drawn from neck veins draining the glands in order to identify the location of an autonomously functioning adenoma.

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E. Indirect Indications of PTH Activity

1. % tubular reabsorption of phosphate ( % TPR ) is decreased as a direct effect of PTH on the kidneys and, along with the consequently depressed serum phosphate, is one of the characteristic findings in primary (or ectopic) hyperparathyroidism. The value is determined from measurements of phoshate and creatinine on a serum and a spot urine specimen:

% TPR = 100x[ 1 - (phos.u/phos.s)/(creat.u/creat.s)]

2. The serum Cl-/phosphate ratio increases from a normal maximum of about 35 up to as high as 80 in cases of primary (or ectopic) hyperparathyroidism. Decreased serum phosphate, as a consequence of decreased renal phosphate reabsorption, is one of the characteristic findings in hyperparathyroidism. A mild to moderate metabolic acidosis is also typically associated with hyperparathyroidism. The anion gap is normal so that serum Cl- is elevated by the amount that HCO3 is depressed. The ratio provides a simple means to justify a suspicion of hyperparathytroidism and to conduct more definitive testing.

3. Urine cyclic AMP increases as a consequence of the interaction of PTH with renal epithelial cell membrane receptors and its intracellular formation as the "second messenger". The interaction of PTH-like protein with the receptors has the same effect.

Cyclic AMP is measured by radioimmunoassay in a spot urine specimen and, along with values for serum and urine creatinine, results are expressed with respect to GFR,
ie:

cyclic AMP excretion rate =
100x(urine cyclic AMP concentraation)/(Cru/Crs)

The test is most useful when PTH is found to not be elevated, but all other test results are consistent with hyperPTH. Elevated cyclic AMP excretion rate and depressed or normal serum PTH concentration strongly suggests ectopic elaboration of a "PTH-like substance".

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Reviewed: March 29, 2001