Page 144 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

134 ELECTROLYTE DISORDERS

Recently, portable clinical analyzers have been devel- of iCa. When zinc heparin is used as an anticoagulant,
oped for cage-side analysis of iCa concentration. These iCa concentration is overestimated most likely
analyzers use a disposable cartridge containing an because of a decrease in pH, which displaces calcium
impregnated biosensor for iCa and other analytes. Hepa- from proteins. 338,340 Lithium heparin causes an
rinized whole blood is used for analysis, but caution underestimation in iCa concentration, 338 and an electro-
should be exercised when interpreting these results. lyte-balanced heparin may underestimate or overestimate
Ionized calcium concentrations in dogs are typically iCa concentration depending on whether hypocalcemia,
0.05 to 0.26 mmol/L lower, and 0.05 to 0.14 mmol/L normocalcemia, or hypercalcemia is present. The amount
lower in cats, when heparinized whole blood is of heparin used is critical in the measurement of iCa in
compared with serum iCa measurement. 227 The greatest blood. Using syringes containing a premeasured quantity
underestimation of iCa concentration occurred when of lithium heparin or electrolyte-balanced heparin, iCa
serum iCa concentrations were greater than 1.3 mmol/ measurement was underestimated when a less than
L. When iCa concentration in heparinized whole blood recommended quantity of blood was collected for analy-
was measured using both ion-selective electrode sis. 338,339 When using heparinized whole blood for mea-
methodology and portable clinical analyzer methods, surement of iCa concentration, it is imperative to collect
the correlation (r) was only 0.71. 398 The portable clinical the same volume of blood for each sample to avoid the
analyzer method resulted in an iCa concentration that dilutional effects of heparin. Syringes containing a
was approximately 2.6% lower than that measured with premeasured amount of dry heparin are preferable to
an ion-selective electrode. 334 However, in a study of dogs coating a syringe manually with an unknown and variable
and horses, there were no differences in iCa quantity of liquid heparin.
concentrations using heparinized whole blood measured Ionized calcium and pH are more stable in serum than
with an ion-selective electrode and portable clinical ana- in whole or heparinized blood. The analysis of serum
lyzer. 337 Because the quantity and type of heparin used eliminates the potential interference of heparin and allows
and volume of blood collected also have an effect on a longer storage period before analysis. Silicone separator
iCa measurement, it is best to establish a rigid protocol tubes should not be used; the iCa concentration was
for blood collection when using a portable clinical ana- increased in serum separated by use of silicone separator
lyzer. Reference ranges should also be established for tubes because of release of calcium from the silicone
the analyzer using this standard protocol. gel. 322 Measured iCa in canine and equine sera was stable
after storage for 72 hours at 23 Cor4 C and for 7 days

Sample Handling Techniques at 4 C. 514,515 Use of serum collected anaerobically and

Concentration of iCa can be determined in samples han- stored at 4 C allows sufficient time for shipment to a

dled under both anaerobic and aerobic conditions. The reference laboratory for anaerobic measurement of
most precise determination of iCa concentration and iCa and pH.
physiologic pH requires that samples be collected and Ionized calcium may also be accurately measured in
processed anaerobically to ensure that no increase in samples handled aerobically. Mathematical formulas have
pH occurs because of loss of CO 2 . The pH of blood or been developed to correct the iCa concentration in
serum has a significant effect on serum iCa concentration. samples exposed to air (with increased pH) to the actual
Acidic pH favors dissociation of calcium from protein and pH of the patient or to a pH of 7.4. 331,400 In a study of
increases the amount of iCa in the sample. Alkaline pH serum samples from 61 dogs and 21 cats, there was good
occurs with loss of CO 2 and favors calcium binding to correlation between iCa measured anaerobically and
protein, thus decreasing the amount of iCa. Mixing again aerobically after shipment to a diagnostic laboratory
serum with air results in increased pH and decreased (Schenck and Chew, unpublished observations). These
measured iCa concentration because of loss of CO 2 from pH correction formulas are species specific, and formulas
the sample. 515 Exposure to air in partially filled developed in humans should not be used. A mathematical
serum tubes also can affect iCa concentration; tubes that correction formula should be derived for each species in
were only 25% or 50% filled had 0.07 or 0.04 mmol/L each laboratory setting. Although not as precise as anaer-
lower concentrations of iCa when compared with obic measurement, aerobic measurement under proper
measurements from tubes that were 100% filled. 587 laboratory conditions offers a diagnostically accurate
Ionized calcium can be measured in whole blood or methodology for iCa determination with simplified
heparinized plasma, but measurement is problematic. shipping and handling requirements.
Heparinized canine blood provided stable iCa Some iCa analyzers will automatically mathematically
measurements when stored up to 9 hours at 4 C, but manipulate the iCa concentration and actual pH value

pH was significantly increased after 3 hours. 553 In prac- of the sample and yield an adjusted value for iCa concen-
tice, it may be impossible to analyze the sample within this tration that theoretically would occur at a pH of 7.4.
period. The amount and type of heparin used for whole These correction formulas were developed for use in
blood or plasma samples also affect the measurement humans and should not be used in animals. When using