Calcium - plasma or serum

Last updated: Saturday, 13, March, 2004

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Item Process
Specimen

Total and corrected calcium: 5 mL blood in lithium heparin or plain tube, minimising venostasis. Ionised calcium: 2 mL blood in an anaerobic plain tube, or a special, balanced heparin tube. Consult laboratory.

Method

Total calcium: spectrophotometry, atomic absorption spectrophotometry. Corrected calcium algorithms vary, but typically:

Corrected calcium (mmol/L) = total calcium (mmol/L) + 0.02 (40 - albumin [g/L].

Ionised calcium - ion selective electrode.

Reference Interval

Total calcium:          2.10-2.60 mmol/L

Corrected calcium:   2.15-2.60 mmol/L

Ionised calcium:       1.16-1.30 mmol/L

Application

(i) Diagnosis of hypercalcaemia. Investigation of patients with clinical features of hypercalcaemia or other features of hyperparathyroidism; malignancy esp lung, multiple myeloma, kidney, bony metastases; sarcoidosis: vitamin D or vitamin A toxicity.

(ii) Diagnosis of hypocalcaemia. Investigation of patients with clinical features of hypocalcaemia or other features of hypoparathyroidism, renal failure, osteomalacia or rickets. Evaluation of patients after thyroid or parathyroid surgery, or during massive blood transfusion.

Interpretation

Total calcium should not be used for evaluation of patients. In most situations, corrected calcium is used. Ionised calcium is required if complexed calcium is likely to be very high (eg during massive transfusion), if pH is abnormal or if an abnormality in calcium is marginal. Artefactual decrease in calcium occurs if EDTA, unbalanced heparin or oxalate is used as an anticoagulant. See Table 1.

Reference

Endres DB and Rude RK. In: Burtis CA and Ashwood ER eds. Tietz Textbook of Clinical Chemistry. 2nd ed. WB Saunders 1994. Marx SJ. Hyperparathyroid and hypoparathyroid disorders. N Engl J Med. 2000 Dec 21;343(25):1863-75. Review.