Page 178 - Basic Monitoring in Canine and Feline Emergency Patients

measured total calcium levels will be (falsely) color of the serum. Therefore, icterus, lipemia,
increased. and hemolysis will alter colorimetric testing and
VetBooks.ir iCa falsely increase phosphorus levels. Similarly,
hyperproteinemia (especially when due to increased
the serum that falsely elevate serum phosphorus
Ionized calcium is only measured by ion-selective serum globulins) will lead to opaque precipitates in
electrodes. Historically, other positively charged levels detected by colorimetric testing. In addition,
electrolytes such as magnesium, lithium, and glassware contaminated with detergents and even
potassium would interfere with the measurement medications can lead to color changes or precipi-
of iCa, causing falsely increased iCa levels. tates in the serum that alter colorimetric testing and
However, newer ion-selective electrodes are less falsely increase phosphorus levels.
likely to have interference and present more reli- Hemolysis of the blood sample will also release
able measures of iCa, minimizing the chances that phosphorus into the serum from the hemolyzed
magnesium, sodium, or administration of sodium cells, increasing the amount of serum phosphorus
or potassium bromide will falsely increase iCa measured. Long-term storage of blood samples can
concentrations. Another reason for falsely also falsely increase phosphorus levels as phospho-
increased iCa levels is release of calcium from the rus leaches out of red and white blood cells and
silicone separator in blood tubes. Follow your thrombocytes into the sample. This is especially
analyzer’s instructions for the appropriate tube common in situations with larger than normal
type to minimize such changes but often lithium numbers of cells in the blood (e.g. thrombocytosis
heparin tubes are used. or leukocytosis).
Other medications may decrease iCa levels by Conversely, mannitol will draw water into the
binding calcium that would normally be in the serum and dilute phosphorus levels (true decreases
ionized (free) form. These medications include in serum phosphorus) or it can interfere with phos-
acetaminophen, salicylates (i.e. aspirin), and acetyl- phorus binding to molydate (the dye used in colori-
cysteine. Heparin mixed with whole blood (as metric testing) and falsely decrease the measured
opposed to serum) can lower iCa levels by 0.05– phosphorus levels. Recent carbohydrate-rich food
0.26 mmol/L in dogs and 0.05–0.14 mmol/L in intake will also decrease circulating serum phos-
cats. This occurs exclusively in inappropriately phorus because the resulting insulin release in
heparinized samples when too much heparin is response to the carbohydrates will push phospho-
combined with too little blood. Common situations rus transiently into cells, creating a falsely low
would include not putting enough blood in a pre- serum phosphorus at the time of measurement.
heparinized blood gas syringes or adding unmeas- Actual circulating phosphorus levels can be meas-
ured amounts of heparin to a syringe. The ured after a fasting period to remove the effect of
appropriate amount of liquid heparin to add to a insulin release on phosphorus.
syringe when testing for iCa is 10 IU/mL.
Changes in blood sample pH can also affect
measured iCa levels. Acidemia causes dissociation
of calcium from proteins and falsely elevates iCa 8.6 Case Studies
levels. In contrast, alkalemia can lead to increased
Ca binding to proteins, falsely lowering iCa levels. Case study 1: The hypernatremic cat
The most common cause of erroneous alkalemia An approximately 10-year-old female spayed
occurs when blood samples are stored exposed domestic long-haired cat named Norah was pre-
to air for long periods of time. Long-term exposure sented to the hospital for evaluation. She had been
to air will cause a decrease in carbon dioxide levels as adopted and spayed approximately 5 weeks prior
it is lost to the environment, resulting in alkalemia. to presentation by her current owners. Since her
spay, she had lost three pounds and had never
eaten or drunk well. She also had a spay site infec-
Phosphorus
tion immediately after the surgery that resolved
Phosphorus is measured by a colorimetric test with antibiotic therapy administered 1 and 4
and is therefore affected by factors that alter the weeks postoperatively. For the 4 days prior to

170 E.J. Thomovsky