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Complementary Tests
Urea concentration should usually be determined in conjunction with measurements of creatinine,
inorganic phosphate, total protein, albumin, and a complete urinalysis. Urea concentration is
influenced by high-protein diet rather than creatinine.
Reaction Sequence
Calcium (Ca)
Calcium is an essential element that is involved in many body systems. These include the skeleton,
enzyme activation, muscle metabolism, blood coagulation, and osmoregulation. In the blood,
calcium exists in ionized and protein bound forms. Factors governing the total plasma, whole blood,
or serum concentration are complex and include interaction with other chemical moieties, proteins,
and hormones.
Calcium, phosphorus, and albumin metabolism are interdependent.
Principal Reason for Performing the Test
As an indicator of certain neoplasias, bone disease, parathyroid disease, eclampsia, and renal
disease.
Most Common Abnormalities Indicated by the Test
Increased calcium—hypercalcemia of malignancy (due to tumor release of PTH-like substances),
spurious.
Decreased calcium—potential renal failure with resultant hyperphosphatemia, dietary, spurious.
Sample Type and Precautions
Remove plasma or serum promptly from the cells or clot. If plasma is being collected, use only
lithium heparinized samples.
Centrifugation should take place quickly after the sample has been drawn. The sample should
not be exposed to the air for long periods. Glassware must be scrupulously cleaned to avoid
contamination by sources of calcium (e.g., detergents). Prolonged contact with the clot may lead to
lowered calcium values due to dilution by red blood cell water.
Do not use tubes containing fluoride, oxalate, citrate, or EDTA as these agents will cause significant
negative interference due to calcium chelation.
If analysis cannot be performed within 4 hours, the sample should be removed from the red blood
cells and refrigerated in a tightly stoppered container at 2°C–8°C (36°F–46°F) for short-term storage
(up to 24 hours). The sample should not be frozen. The sample must be allowed to reach room
temperature before analysis.
Complementary Tests
Calcium should be determined in conjunction with measurements of inorganic phosphate, albumin,
total protein, and glucose. Ionized calcium measurement will provide more specific information
related to the physiologic form of calcium.
Reaction Sequence
Appendices
