Construction of new formula for estimating corrected Caconcentration the modified bromocresol purple method

†Naohito Ishii, PhD*

Editorial
Lab Med Int 2023; 2(2): 17

†Correspondence: Professor Clinical Chemistry Laboratory, Department of Medical Laboratory Sciences, School of Allied Health
Sciences, Kitasato University, Kanagawa 252-0373, Japan. E-mail: naohito3″@”ahs.kitasato-u.ac.jp
*Associate editor of Laboratory Medicine International; Clinical Chemistry Laboratory, Department of Medical Laboratory Sciences, School of Allied Health Sciences.

See article volume 2: 19-24

In this issue of Laboratory Medicine International, Ishigaki et al.’s paper :“A new equation for estimating iCa over a wide range of serum albumin concentrations measured by the modified bromocresol purple method”, has been published. Construction of a new formula for estimating the corrected Ca(cCa) concentration using a modified bromocresol purple(BCP) method is an interesting and clinically important development.
Ionized Ca(iCa) is 50% of the total serum, 40% exists as protein-bound forms, mainly albumin (Alb), and 10% exists as chemically bound forms¹⁾. Therefore, serum Ca concentration measurements must be corrected by the serum Alb concentration.
Since the amount of protein-bound Ca decreases in hypoalbuminemia, the serum Ca concentration may not reflect the actual Ca concentration. For this reason, Payne’s formula(cCa(mg/dL)=tCa(mg/dL)+4–Alb(g/dL)) is most commonly used in patients with hypoalbuminemia in Japan^2-4). However, because the cCa concentration is only an estimated value of the Ca concentration, it does not reflect the true Ca concentration. A characteristic of iCa is that as the pH decreases, the number of Alb-binding sites in iCa decreases and the ionization rate of Ca in serum increases⁵⁾.
Ishigaki et al. point out three problems with Paine’s formula: 1) the bromocresol green(BCG) method has cross-reactivity with proteins other than Alb(globulin, C-reactive protein, etc.), 2) iCa is not considered an index , and 3) it is applicable only when Alb concentrations are low.
In this study, a new formula was constructed to estimate the cCa concentration over a wide range of serum Alb concentrations using a modified BCP method corrected to pH 7.4(cCa(mg/dL)=tCa(mg/ dL)+0.4(4.6–Alb(g/dL)).

Reference