Calcium, Total (Ca), Plasma / Urine / Fluid

 

Specimen requirement

Plasma

Urine

Fluid

Adult patient:

 

 Heparin gel tube

 

Minimum blood volume: 5 mL

 Spot urine:

Plain bottle

Minimum volume: 10 mL

24 Hr urine:

U2 bottle

 

 Spot fluid:

Plain bottle

Minimum volume: 10 mL

 

Paediatric patient:

 

Paediatric heparin tube

 

Number of vials: 1

 

Minimum blood volume: 1.3 mL

Authorisation code required

No

 No No

24 Hr available service

Yes

 

(Call extension: 3353 for express result)

Spot urine: Yes

 

24 Hr urine: No

 

No

Method

Spectrophotometry

Reference interval

Plasma: 2.15 - 2.55 mmol/L

 

24 Hr urine: 2.0 - 7.4 mmol/day

 

(Source: Robert WL et al. Reference Information for the Clinical Laboratory. In Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 4th Edn. Burtis CA, Ashwood ER, Bruns DE eds. Elsevier Saunders 2006)

Clinical indication

Plasma calcium:

  1. Diagnosis of hypercalcaemia. Investigation of patients with clinical features of hypercalcaemia or other features of hyperparathyroidism; malignancy especially lung, multiple myeloma, kidney, bony metastases; sarcoidosis; vitamin D or vitamin A toxicity
     

  2. 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

Urinary calcium:

  1. Investigation of renal calculi
     

  2. Diagnosis of hypocalciuric hypercalcaemia

Result interpretation

Plasma calcium:


In most situations, corrected calcium is used for evaluation of patients.

 

Adjusted calcium (mmol/L)

= {35.7 – Albumin concentration (g/L)} / 74.9 + Total calcium measured (mmol/L)

Albumin-adjusted calcium formula has not been validated for plasma albumin (BCP dye-binding method) concentration < 24 or > 50 g/L.  Please request ionised calcium for the investigation of calcium disorder.

Ionised calcium is required if complexed calcium is likely to be very high (e.g. 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.

 

Urinary calcium:

 
Increased excretion seen with high dietary intake of calcium, increased mobilisation of calcium from bone, and most conditions associated with hypercalcaemia.

 

Hypercalciuria is a risk factor for renal calculi.

Measurement of uncertainty

See table.

Frequency of measurement

Daily