Laboratory Investigations of Parathormone (PTH)
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p345-346
2025-09-25
206
Calcemia
Total calcemia is determined by colorimetric methods, which provide accurate and reproducible performance under well- controlled operating conditions; the assay does not require fasting or preparation for testing. However, if the patient is taking drugs that may interfere with calcemia values (vitamin D3, lithium), it is advisable to suspend their administration before performing the assay temporarily. In the presence of hypoalbuminemia, the above formula is required to obtain the corrected calcemia. Ionized calcium should be determined in selected cases (malabsorption, chronic disabling diseases).
Albumin
It is measured by separative methods (serum protein electrophoresis) or techniques that directly evaluate the concentrations, using specific reagents (methods using dyes; fluorimetric methods). Serum protein electrophoresis is the most reliable among the separative methods. The albumin assay does not require test preparation or fasting.
Renal Function
Creatinine measurement and clearance are used to assess kidney function.
Magnesium
It can be performed by atomic absorption spectrophotometry or colorimetric enzymatic methods, which are widely used in clinical practice. The reference values of plasma magnesium are 1.7–2.1 mg/dL.
Phosphate
It is performed by colorimetric enzymatic techniques and requires fasting from the previous 12 hours.
Serum or plasma should be separated as soon as possible from the corpusculate, which is very rich in phosphate esters. The reference values of plasma phosphates are 2.5–4.5 mg/ dL. Phosphaturia in 24-hour urine is of little value if not related to dietary intake and tubular reabsorption; therefore, phosphate clearance, corrected with creatinine clearance (400–1000 mg/24 h), is more frequently calculated.
PTH
PTH determination is a diriment test in the etiologic diagnosis of hypercalcemia (for the differential diagnosis between PTH-dependent and non-PTH-related forms) and the differential diagnosis between hypoparathyroidism and pseudo- hypoparathyroidism. PTH is determined by immune chemiluminescence; the fragment tested is PTH 1-84, or intact PTH. Reference values are between 10–55 pg/mL.
Vitamin D3
The most commonly assayed vitamin D3 metabolite is 25-(OH)-vitamin D3, which indicates available stores related to dietary intake and synthesized from the skin. It is appropriate to measure 1,25-(OH)2-vitamin D3 for the differential diagnosis of vitamin D3-dependent and vitamin D3- resistant rickets. Reference values of 25-(OH)-vitamin D3 are shown in Table 1. The reference values of 1,25-(OH)2- vitamin D3 are: 20–60 pg/mL.

Table1. Values of 25–(OH)-vitamin D3 and their interpretation
PTHrP
It is performed when a neoplastic origin of hypercalcemia is suspected. Values below 1 pmol/L are suggestive of tumor etiology. The PTHrP assay has limited uptake in clinical practice.
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