Keywords: Ca


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


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:          See Table 6.

Corrected calcium:   See Table 6. 

Ionised calcium:       1.16-1.30 mmol/L


(i) Diagnosis of Hypercalcaemia. Investigation of patients with clinical features of hypercalcaemia or other features of Hyperparathyroidism; malignancy esp lung, Plasma cell 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.


Total calcium may be affected by serum albumin. In most situations, albumin-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.


Endres DB and Rude RK. In: Burtis CA and Ashwood ER eds. Tietz Textbook of Clinical Chemistry.2nd ed. WB Saunders 1994.

Marx SJ. N Engl J Med. 2000 Dec 21;343(25):1863-75. Review.