Considerations when selecting a laboratory test:
(1) The test measures a disordered process that is occurring in the patient (bone resorption, bone mineralization, bone formation).
(2) The test is specific (not affected by other processes). Some measures such as urinary hydroxyproline and serum alkaline phosphatase are nonspecific. A specific test such as the bone specific isoenzyme of alkaline phosphatase are more informative.
(3) The test is responsive - a change in the test value reflects a change in the underlying process.
(4) The test has a low analytical coefficient of variability.
(5) Testing is done to minimize the impact of normal physiologic variation during the day. Ideally the specimen should be collected at the same time of day each time.
(6) The test uses a specimen that does not contribute to the variation in results. A serum test is preferred to a 24 hour urine test because of the difficulty in accurate collection of the latter. Urine samples are also subject to the patient's hydration status, which can be compensated for by adjusting for the urine creatinine excretion.
(7) The test is cost-effective.
Tests used to monitor mineralization:
(1) serum and urine calcium
(2) serum and urine phosphate
(3) 25-hydroxy and 1,25-dihydroxy vitamin D
Tests for bone formation:
(1) osteocalcin
(2) bone specific alkaline phosphatase
(3) Type I collagen propeptides
Tests for bone resorption:
(1) Type I collagen telopeptides (N-terminal, C-terminal, others)
(2) hydroxylysine, galactosyl hydroxylysine, glucosyl galactosyl hydroxylysine
(3) pyridinoline and deoxypyridinoline (collagen pyridinium crosslinks)
(4) tartrate resistant acid phosphatase
(5) parathyroid hormone
