Page 142 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

132 ELECTROLYTE DISORDERS

source of calcium in addition to the internal mobilization TABLE 6-1 Normal Serum
from bone. Hypocalcemia increases transcription of the Concentrations
PTH gene and synthesis of PTH mRNA, enhancing the
ability of the chief cells to produce PTH. This effect also Dog Cat
occurs within hours of hypocalcemia. Over days or weeks
of hypocalcemia, further increases in PTH secretion are Total Calcium
achieved largely by hypertrophy and hyperplasia of chief mg/dL 9.0-11.5 8.0-10.5
cells in the parathyroid gland. 497 In addition, the propor- mmol/L 2.2-3.8 2.0-2.6
tion of chief cells actively synthesizing PTH is increased. Ionized Calcium
mg/dL 5.0-6.0 4.5-5.5
NORMAL HOMEOSTATIC mmol/L 1.2-1.5 1.1-1.4
RESPONSE TO Parathyroid Hormone (PTH) 2-13* 0-4*
Intact (pmol/L)
HYPERCALCEMIA N-terminal (pg/mL) 5-55 8-28
Parathyroid Hormone Related <1.0* <1.0*
Most of the effects that occur during hypercalcemia are protein (PTHrP) (pmol/L)
the opposite of those described earlier for hypocalce- (intact or N-terminal)
mia. 478 Hypercalcemia results in decreased PTH secre-
tion, increased intracellular degradation of PTH in 25-Hydroxyvitamin D 60-215* 65-170*
chief cells, and decreased PTH synthesis. Increased calci- (calcidiol) (nmol/L)
1,25-Dihyroxyvitamin D
tonin secretion is stimulated in an attempt to minimize
(calcitriol) (pg/mL)
the magnitude of hypercalcemia. In addition, hyperplasia
of C cells in the thyroid gland results if the hypercalcemic Adults 20-50 20-40
20-80
60-120
10-12-week old
stimulus is sustained, but this mechanism is ineffective for
controlling hypercalcemia because of the transitory effect *Data from Endocrine Diagnostic Section, Diagnostic Center for
of calcitonin on osteoclastic bone resorption. 420,484 Population and Animal Health, Lansing, Michigan.
Calcitriol synthesis is decreased both through direct inhi-
bition by iCa and as a result of decreased stimulation
because of decreased PTH concentration. tCa measurement overestimated normocalcemia and
underestimated hypocalcemia, 519 and in cats, hypercalce-
DIAGNOSTICS mia and normocalcemia were underestimated, and hypo-
calcemia was overestimated when using serum tCa
Table 6-1 lists the normal values for serum tCa, 115 iCa, 114 concentration to predict iCa status. 518
PTH, 404,577 PTHrP, 489 and vitamin D metabolites that
are useful in the diagnostic workup of patients with Analytical Methods
calcium disorders. 478 Fasting serum or heparinized plasma samples should be
submitted for analysis. Oxalate, citrate, and ethylenedia-
TOTAL CALCIUM minetetraacetic acid (EDTA) anticoagulants should not
Despite the fact that only the iCa fraction is physiologi- be used because calcium is bound to these chemicals
cally active, the calcium status of animals is usually initially and becomes unavailable for analysis. 623
based on evaluation of the serum tCa concentration. Serum tCa concentrations vary with the method used.
Measurement of tCa concentration is more readily avail- Isotope dilution with subsequent mass spectrometry
able than iCa measurement, but it does not always accu- constitutes the definitive method for calcium measure-
rately reflect the iCa concentration of the patient. The ment but is not readily available. 200 For clinical determi-
serum tCa concentration has been assumed to be directly nation of serum tCa concentration, simple colorimetric
proportional to iCa, but in many clinical conditions, this reactions and spectrophotometry are usually employed
may lead to erroneous interpretation of laboratory data. using automated or manual methods. Ortho-
In humans with disorders of calcium balance, measure- cresolphthalein complexone is a metal dye that is com-
ment of serum tCa concentrations failed to predict serum monly used to form a color complex with calcium. This
iCa concentrations in 31% of all patients 566 and in 26% of method is accurate and reproducible. 200 Hemolysis can
patients with renal disease. 88 In 1633 canine samples, result in formation of an interfering hemoglobin-chro-
diagnostic disagreement between serum iCa and tCa mogen complex that falsely increases measured calcium
was 27%, and in dogs with CRF, this disagreement was concentration. High concentrations of bilirubin falsely
36%. 519 In cats, serum iCa concentrations were only decrease, and acetaminophen and hydralazine falsely
moderately correlated with serum tCa concentrations, 142 increase serum tCa concentration. Lipemia can result in
and a 40% diagnostic disagreement between serum iCa spuriously high calcium concentrations, 380 with values
and tCa measurement was noted in 434 cats. 518 In dogs, exceeding 20 mg/dL in some instances of severe lipemia.