Correct calcium by albumin level

Q: Correct calcium level is usually calculated by albumin level. What percentage of calcium is usually bound to albumin? (select one)

A) 15-25%

B) 40-45%

C) 60-75%

Answer: B

The objective of this question is to highlight the fact that conventional formulae used at the bedside to correct calcium given albumin level is not a reliable science as only 40-45% of Ca is bound to proteins, primarily albumin.

The formula usually used is 

Corrected [Ca]  =  Measured total [Ca] + (0.8  x  (4.0 - [Alb]))

Studies have shown that this widely used formula is very unreliable, particularly for ICU patients, and patients with renal insufficiency.

The gold standard method is to measure the ionized calcium.

On the same token, hyperalbuminemia may cause pseudo-hypercalcemia.

#electrolytes

References:

1. Gauci C, Moranne O, Fouqueray B, et al. Pitfalls of measuring total blood calcium in patients with CKD. J Am Soc Nephrol 2008; 19:1592.

2. Lian IA, Åsberg A. Should total calcium be adjusted for albumin? A retrospective observational study of laboratory data from central Norway. BMJ Open 2018; 8:e017703.

3. Ridefelt P, Helmersson-Karlqvist J. Albumin adjustment of total calcium does not improve the estimation of calcium status. Scand J Clin Lab Invest 2017; 77:442.

4. Smith JD, Wilson S, Schneider HG. Misclassification of Calcium Status Based on Albumin-Adjusted Calcium: Studies in a Tertiary Hospital Setting. Clin Chem 2018; 64:1713.

5. Slomp J, van der Voort PH, Gerritsen RT, et al. Albumin-adjusted calcium is not suitable for diagnosis of hyper- and hypocalcemia in the critically ill. Crit Care Med 2003; 31:1389.