NAPLEX® Review Question of the Week: Calcium Correction
AC, a 48-year old male, is in the hospital for a recent diagnosis of decompensated liver failure.
AC’s laboratory findings and vitals are reported below:
Labs/Vitals:
Ht: 5’10'' ft
Wt: 189 lbs
BP: 135/80mmHg
HR: 79 BPM
Temp: 98.2 F
Glucose: 103 mg/dL (70-130 mg/dL)
Scr: 0.5 mg/dL (0.5-1.2 mg/dL)
Na: 136 mEq/L (135-145 mEq/L)
Cl: 98 mEq/L (95-105 mEq/L)
K: 3.5 mEq/L (3.5-5 mEq/L)
Ca: 7.4 mg/dL (8.5-10.5 mg/dL)
Mg: 2.1 mg/dL (1.7-2.2 mg/dL)
Albumin: 2.2 g/dL (3.5-5.5 g/dL)
What would AC’s corrected Ca2+ be? Round your response to the nearest tenths place.
Answer with rationale:
Our bones store nearly 99% of the body's calcium, which is the most abundant mineral in the body. The remaining 1% (around 8.5-10.5 mg/dL) is free calcium located in the bloodstream. About 40% of this calcium is bound to albumin. Calcium binds to albumin for transport, storage, and to regulate the amount of free calcium available. When calcium binds to albumin it ensures there is not only a reservoir of bound calcium but also an appropriate amount of free calcium available for the body to use.
In cases of hypoalbuminemia, there is less albumin available to bind calcium, which leads to inaccurate interpretations of true calcium levels. To address this issue, healthcare providers often rely on a "corrected calcium" calculation to account for the effects of hypoalbuminemia on total calcium levels.
There is some controversy regarding how accurate this calculation may or may not be. Another common approach is to obtain a direct ionized or "free" calcium level. In intensive care unit patients receiving blood gas measurements, this can often be obtained which is a very accurate measure of free calcium, with levels around 4.5- 5.6 mg/dL or 1 mmoL/L representing a normal level.
Here is the correct approach for determining AC's corrected calcium. It is important to note that one of the question types for the NAPLEX is constructed-response, where you may have to "fill in the blank" with an answer as directed in the question which could be a calculation.
Corrected calcium = (0.8 x [4 – patient’s albumin]) + measured calcium
Corrected calcium = (0.8 x [4 – 2.2 g/dL]) + 7.4 mg/dL
Corrected calcium = 8.8 mg/dL
Answer C is correct.
NAPLEX Content Domains Covered
Domain 1: C. 1 – Patient parameters and laboratory measures
Dr. Christopher M. Bland is the Albert W. Jowdy Professor in Pharmacy Care at the University of Georgia College of Pharmacy at the Southeast GA campus in Savannah, GA. Dr. Bland has nearly 25 years of academic and clinical experience both in the inpatient and community patient care settings. He is a Fellow of both the Infectious Diseases Society of America as well as the American College of Clinical Pharmacy and has been named Teacher of the Year at the University of Georgia College of Pharmacy. He is co-author of the book "InteGREAT: A Guidebook for Creating Great Healthcare Teams."
He is co-founder of the Southeastern Research Group Endeavor, SERGE-45, with nearly 100 practitioners across 14 states involved. Dr. Bland serves as Associate Editor for the NAPLEX Review Guide 4th edition as well as upcoming 5th edition, as well as the Editor-In-Chief for the Question of the Week. He has provided hundreds of hours in live, interactive NAPLEX reviews for more than 10 Colleges/Schools of Pharmacy over the course of his career.
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