Page 145 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Calcium: Hypercalcemia and Hypocalcemia 135

anaerobically collected samples, corrected iCa concept that the iCa fraction is tightly regulated and
concentrations have not been advocated for use in represents the biologically active fraction of serum
humans because insight into the pathophysiology of the calcium.
patient is gained by evaluation of the in vivo iCa concen- Complexed and protein-bound calcium fractions have
tration and pH. 199 This may be especially true for patients not been assessed in metabolic disorders associated with
with renal disease. 498 If anaerobic sampling is possible abnormal calcium concentrations. Measurement of the
(typically in an in-house setting), there is no necessity protein-bound and complexed calcium fractions in addi-
or benefit in correcting the iCa concentration to a pH tion to the iCa fraction may facilitate detection of disease
of 7.4. Only when samples are handled aerobically is there processes that affect calcium metabolism. In dogs with
a need for correction to a standard pH. In one study, iCa CRF, two subgroups have been identified based on cal-
measurement was stable in aerobically handled serum for cium fractionation. Dogs with normal to elevated serum

up to 48 hours at 4 C; however, in this study samples tCa concentrations had a significantly higher concentra-
were undisturbed for this period so they had minimal tion of circulating complexed calcium as compared with
mixing with air. 68 During shipment to a laboratory, those dogs with low concentrations of tCa, even though
serum mixes considerably with air, causing a significant there was no difference in iCa or protein-bound calcium
decrease in iCa due to an increase in pH. Thus, it is not between groups. 517 Further studies are needed to deter-
recommended to measure iCa in shipped aerobically han- mine whether prognosis or effectiveness of therapy differs
dled samples without adjustment to a standard pH of 7.4. between these groups.
Normal Values PARATHYROID HORMONE
The range for serum iCa concentration in normal dogs PTH circulates predominantly as intact PTH (1-84) and
and cats varies among laboratories but is approximately carboxyl-terminal fragments. Only intact PTH is biologi-
5.0 to 5.8 mg/dL (1.25 to 1.45 mmol/L) in adult cally active, and it is best to measure this form in serum or
dogs 516 and 4.6 to 5.4 mg/dL (1.15 to 1.35 mmol/L) plasma. Samples should be stored and shipped frozen to
in adult cats. 142 An effect of aging has been observed prevent degradation of intact PTH. Stability is best in
in both the dog and cat. Young dogs and cats (up to plasma collected with EDTA, but serum is adequate if
2 years of age) have serum iCa concentrations that are stored frozen after separation from blood. Because of
0.1 to 0.4 mg/dL higher than those reported in older the sequence homology of human and animal PTH,
animals. 142,385 Normal values should be established for commercial assays developed for humans have been used
each laboratory based on the age of the animal, type of successfully for some veterinary species. 117 An amino-
sample, and analyzer used. terminal-specific radioimmunoassay (RIA) was used for
more than 50 mammalian species but is no longer com-
Fractionation of Serum Calcium mercially available. 402 A two-site immunoradiometric
In addition to measuring the ionized concentration in the assay (IRMA) for intact human PTH has been validated
serum, the protein-bound and complexed fractions of in the dog and cat. 26,577 Normal values for serum PTH
calcium can be quantified using fractionation techniques. concentration using this assay were 2 to 13, 0 to 4, and
Ionized calcium and complexed calcium are diffusible, 0 to 2 pmol/L in the dog, cat, and horse, respectively
and together are referred to as ultrafilterable calcium. (Endocrine Diagnostic Section, Diagnostic Center for
To separate protein-bound from ultrafilterable serum cal- Population and Animal Health, Lansing, MI). Unfortu-
cium, a micropartition system based on the filtration nately, this assay has been discontinued. The two-site
method has been used. 173,516 The micropartition system assays have not proved useful for measurement of PTH
contains a filter through which ultrafilterable calcium in reptiles. The expected responses of PTH in various
(complexed and ionized) passes. It is important that conditions will be discussed later (see Hypercalcemia
serum be collected anaerobically before ultrafiltration and Hypocalcemia sections).
to allow accurate measurement of the calcium fractions The two-site IRMA measures both the intact PTH-
and to prevent changes in serum pH. (1-84) and the PTH-(7-84) fragment, because the
Protein-bound, ionized, and complexed calcium amino-terminal antibodies react near the tenth amino
fractions in serum were 34%, 56%, and 10% in normal acid. 74,133,413 A new third-generation IRMA “whole”
dogs 516 and 40%, 52%, and 8% in normal cats, respectively PTH assay has been developed for use in humans that
(Schenck, unpublished observations). Ultrafilterable cal- measures only PTH-(1-84). 204 This new assay could offer
cium (ionized and complexed fractions) in dogs, 516 a better measure of whole PTH, especially in patients with
horses, 255 and cats (Schenck, unpublished observations) secondary hyperparathyroidism because the PTH-(7-84)
accounted for 66%, 63%, and 60% of serum tCa, respec- fragment is increased in these patients. 386 High
tively. The iCa fraction has the smallest variation, with concentrations of carboxyl-terminal PTH fragments,
larger variations occurring in the protein-bound and which occur in cats with CRF, may interfere with intact
complexed fractions. This observation supports the PTH immunoassays. 30 Using ratios of “whole” PTH