Why do labs correct total calcium based on albumin?
A: Ionized calcium is the most accurate test for assessing a patient’s calcium status, but its application remains limited. Clinical laboratories were not able to broadly use the first method developed for measuring ionized calcium because it was based on a bioassay requiring frog tissue. Following the introduction of direct potentiometry with ion-selective electrodes (ISE), the availability and precision of ionized calcium measurement has improved, but preanalytical and analytical challenges still hamper its universal application. These include issues related to sample handling, cost, equipment maintenance, analytical performance, and lack of measurement standardization.
In light of this, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) states that total calcium measurement may be used as a surrogate for ionized calcium in patients who do not have protein and pH abnormalities. Labs can measure total calcium using ISE, atomic absorption spectrophotometry, or photometric methods employing metallochromic indicators or dyes such as o-cresolphthalein complexone and arsenazo III. However, the correlation between ionized calcium and total calcium can be compromised by alterations in albumin concentration, blood pH, elevated levels of drugs or fatty acids bound to albumin, and unusual serum proteins such as monoclonal immunoglobulins. This is a major drawback of total calcium measurement, especially in hospitalized patients.
To overcome this, various nomograms and formulae have been developed to estimate ionized calcium by correcting total calcium for total protein, albumin, globulins, and pH. The most widely used of these is the Payne et al. formula: Adjusted calcium (mmol/L) = Total calcium (mmol/L) + 0.02 [40 – serum albumin (g/L)]. This and other correction formulae were derived by determining the linear regression relationship of serum calcium to albumin concentration in healthy patients.
What are the shortcomings of using correction formulae?
The Payne formula and related equations were derived decades ago using the bromocresol green (BCG) method for albumin measurement. Since then, however, analytical techniques for albumin measurement have changed. BCG overestimates serum albumin because of nonspecific dye binding with many proteins, particularly at low serum albumin concentrations, so a large proportion of labs now use the more effective bromocresol purple (BCP) method. If a lab uses the BCP method, this can affect the performance of BCG-based equations. Studies also show that the Payne formula correlates poorly with ionized calcium in specific patient populations such as critically ill surgical patients, renal failure and hemodialysis patients, primary hyperparathyroidism patients, and very elderly hospitalized patients. For these cases, BCP-specific and other alternative formulae have been developed, but experts recommend that labs use ionized calcium for the most accurate results.
How should labs approach calcium measurements?
Taking all of this into account, there are several ways laboratories can handle calcium measurements. Labs may elect to measure ionized calcium in all samples. In some countries this is standard practice, although technical reasons may hinder this approach. If choosing this route, labs can refer to recommendations provided by the IFCC on using ISE to determine ionized calcium in whole blood, plasma, and serum, as well as on sampling, transport, and storage for this test. These recommendations emphasize rapid analysis of an anaerobic sample placed on ice to counteract various causes of pH alteration, which impacts the concentration of ionized calcium. The IFCC also cautions labs to avoid dilution effects from anticoagulant solutions such as heparin.
On the other hand, if testing all samples for ionized calcium proves to be too difficult, labs might elect to measure ionized calcium only in specific patients for whom there is a clear clinical indication. For all other patients, labs can report albumin-adjusted total calcium, with the caveat that—given the differences in the performance of published correction equations—each laboratory should derive its own equation for correcting total calcium based on albumin.
Tahir S. Pillay, MBChB, PhD, FRCPath, FCPath, is chief specialist, professor, and head of the department of chemical pathology at the University of Pretoria in South Africa. +Email: [email protected]