Calcium Homeostasis and Measurement

Understanding Calcium Fractions

Calcium exists in blood as three distinct fractions: protein-bound calcium, free (ionized) calcium, and complexed calcium, with protein-bound calcium being primarily bound to albumin 1
Only ionized calcium is biologically active and interacts with calcium sensors on cells 1

In advanced chronic kidney disease (CKD), the fraction of total calcium bound to complexes is increased, resulting in decreased free (ionized) calcium levels despite normal total serum calcium levels, particularly problematic in CKD Stage 5 patients 2, 3
A 0.1 unit increase in pH decreases ionized calcium concentration by approximately 0.05 mmol/L because alkalosis enhances binding of calcium to albumin 1
Acidosis increases free calcium by displacing calcium from albumin—a fall in pH of 0.1 unit causes approximately a 0.1 mEq/L rise in ionized calcium 1

In nephrotic syndrome, total serum calcium levels underestimate calcium content in the presence of hypoalbuminemia due to urinary loss of vitamin D-binding protein, and close monitoring of ionized calcium, 25-OH-D3, and PTH levels is recommended 4
The American Journal of Kidney Diseases recommends measuring ionized calcium directly when total calcium is elevated but clinical suspicion for hypocalcemia exists, and maintaining ionized calcium >0.9 mmol/L minimum to prevent cardiac dysrhythmias and coagulopathy 1
The National Kidney Foundation suggests that treatment should be based on ionized calcium levels and symptoms, not total calcium, if ionized calcium is truly low despite elevated total calcium 2, 1

The ionized calcium level for a patient with total serum calcium of 2.72 mmol/L and albumin of 48 g/L is approximately 1.29 mmol/L, which falls within the normal range of 1.15-1.36 mmol/L 5

Corrected calcium (mmol/L) = Total calcium (mmol/L) - 0.0704 × [Albumin (g/L) - 34], as recommended by the K/DOQI Clinical Practice Guidelines 6

Ionized calcium typically represents approximately 45-50% of the total calcium in the extracellular plasma, with the remainder bound to proteins and other molecules in a biologically inactive state 7, 8

Ionized calcium is crucial for the formation and stabilization of fibrin polymerization sites and for platelet-related activities 7
Maintaining normal ionized calcium levels is important for cardiac contractility and systemic vascular resistance 7

The relationship between total calcium and ionized calcium can be affected by pH changes - a 0.1 unit increase in pH decreases ionized calcium by approximately 0.05 mmol/L 7