Estimation of Total Calcium in Serum
and Urine^21
21.1 Theory
Calcium is the most abundant mineral cation in the body which contains 1–1.5 kg of
total body weight in the adults. Over 99% is present in the bones and teeth. A small
fraction is also present outside the skeletal tissue and performs wide variety of
functions. Calcium is vitally important in maintaining the correct conditions for
normal neuromuscular transmission and glandular secretions and for the activity of
enzyme systems. Most of blood calcium is present in plasma. There is almost no
calcium in red blood cells and other intracellularfluid. About 50% of total plasma
calcium exists in ionized form which is functionally most active. About 40% of
plasma calcium is protein bound, and 10% is complexed with citrate, organic acid, or
bicarbonates. About 90% of protein-bound calcium is associated with albumin, and
the remaining 10% is associated with globulins. Ionized and citrate-bound calcium is
diffusible from blood to the tissues, while protein bound is nondiffusible (Fig.21.1).
In the laboratory, all three fractions of calcium are measured together. Calcium binds
to negatively charged sites on proteins which is pH dependent. Alkaline conditions
promote calcium binding and decrease free calcium, whereas acidic conditions
decrease calcium binding and increase free calcium levels.
21.2 Specimen Requirements
Serum/plasma (fasting) sample is required. Take blood with minimal venous occlu-
sion and without exercise. For urine sample, collect 24 h urine in the container
containing 10 ml HCl (6 mol/L) or acidify after collection to pH < 2.0 to dissolve
calcium salts.
#Springer Nature Singapore Pte Ltd. 2018
V. Kumar, K. D. Gill,Basic Concepts in Clinical Biochemistry: A Practical Guide,
https://doi.org/10.1007/978-981-10-8186-6_21
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