Diagnostic Use
Total calcium vs ionised calcium
Ionised calcium (iCa) is the free component (about 50% of total) that is not bound to protein or anions in the form of a salt. iCa is the biologically active fraction, but it is less commonly measured as it is only available on blood gas analyser, which introduces delays, workflow issues and additional costs. Total calcium (tCa) is a good surrogate in the vast majority of cases, except when the concentration of proteins or organic anions is significantly different from normal (e.g. myeloma, citrate anticoagulation).
Corrected or adjusted calcium
The routine test for plasma calcium concentration measures the total calcium (bound plus ionised calcium).
Approximately 50% of plasma calcium is bound to albumin, and this bound fraction is biologically inactive.
Therefore changes in albumin concentration will affect the total calcium concentration without affecting the ionised (active) fraction of plasma calcium.
In order to allow the use of a single reference interval for total calcium, irrespective of albumin concentration, the total calcium can be adjusted using a formula. This is known as the “corrected” or “adjusted” calcium.
The formula used is: [Ca adjusted] = [Ca total] + 0.012 x (39.9 – [albumin])
Interpretation
Increased calcium concentrations: primary and tertiary hyperparathyroidism, malignancies (both with and without bone involvement), sarcoidosis, vitamin D overdosage, dehydration, Paget’s disease (with immobilisation) and genetic disorders of the calcium sensing receptor.
Decreased calcium concentrations: hypoparathyroidism, pseudohypoparathyroidism, chronic renal failure, renal tubular disorders, magnesium deficiency, vitamin D deficiency, prolonged anticonvulsant therapy, acute pancreatitis, rickets and osteomalacia.
Reference Intervals
Units: mmol/L
| Age:
Day 1 to 3 |
1.80 – 2.80
|
| Day 4 to 1yr |
2.10 – 2.80
|
| >1yr: |
2.10 – 2.55
|
This test measures total plasma calcium (includes ionised, protein-bound, and complexed fractions).
Reference ranges above apply to total calcium when serum albumin is normal , and to corrected calcium (adjusted for albumin concentration).
Serum albumin should always be measured in conjunction with total calcium, to enable calculation of corrected calcium.
Prolonged stasis during venepuncture and haemolysis will give falsely high values.
Conversion factors:
- mg/100 mL x 0.25 = mmol/L
- mmol/L x 4 = mg/100 mL
Test Method
Performed in: LabPlus Automation
Principle : Colorimetric
Reagents: Roche assay
Analyser: Roche Cobas
Uncertainty of Measurement
5%