APPROACH TO THE HYPERCALCEMIC PATIENT
In cases of mild (tCa < 13 mg/dl) and moderate (tCa = 13 to 15 mg/dl) hypercalcemia, therapy can be delayed while the diagnosis
is pursued in a logical, stepwise fashion.2 In severely hypercalcemic patients (tCa > 15 mg/dl) or those with a serum calcium-phosphorus product > 70, immediate intervention
to lower serum calcium concentrations may be necessary.6 In such cases, it is imperative to establish a diagnosis quickly, and it is appropriate to investigate all likely causes
In many cases, a database that includes a complete history, a physical examination, retroviral testing, a complete blood count,
a serum chemistry profile, a urinalysis, thoracic and abdominal radiographic examinations, and an abdominal ultrasonographic
examination will establish a preliminary diagnosis. In other cases, specific evaluation of calcium homeostasis may be necessary
(Table 5). The following is a brief review of the standard diagnostic tests, and an algorithm (Figure 1) is also presented as a guide.
1. Physical examination
Since neoplasia is the most likely cause of moderate to severe hypercalcemia, a thorough physical examination is essential,
paying particular attention to the mammary glands, peripheral lymph nodes, and skin. Abdominal palpation may reveal organomegaly,
or thoracic auscultation may point toward pulmonary, mediastinal, or pleural disease. Examine the cervical region carefully,
as parathyroid adenomas are sometimes palpable.7
2. Retroviral testing
ELISA tests for feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) infections are appropriate in any cat
whose retroviral status is uncertain. Always verify a positive ELISA test result with an alternative test, such as an immunofluorescent
antibody (IFA) test. Vaccination against FIV infection will result in positive results with ELISA, IFA, and Western blot tests
for as long as vaccine antibodies persist.
If a hypercalcemic cat is FeLV positive, the index of suspicion for lymphoma is higher. FIV infection does not necessarily
increase the incidence of neoplasia, but it may predispose patients to osteolytic or granulomatous disease.
3. Complete blood count
Anemia suggests chronic inflammatory disease, renal compromise, or myeloinfiltrative disorders. Cats with idiopathic hypercalcemia
or hyperparathyroidism are not anemic, so finding a low hematocrit suggests occult cancer or organ dysfunction. Carefully
evaluate the leukogram for abnormal cell distribution or morphology, which indicates lymphoma or leukemia.
4. Serum chemistry profile
Since hypercalcemia is both a cause and consequence of renal failure, markers of kidney function (blood urea nitrogen [BUN]
and creatinine concentrations) are of particular interest. In general, only severe hypercalcemia is thought to cause renal
injury; mild hypercalcemia in an azotemic patient is more likely to be secondary to kidney disease. It is important to measure
iCa concentrations in azotemic cats before starting a complex work-up, as elevations in tCa concentrations without increases
in iCa concentrations are common in this population.4
Serum phosphorus concentrations often indicate the cause of hypercalcemia. Hyperphosphatemia suggests intrinsic renal failure,
vitamin D toxicosis, or osteolytic disease. In contrast, serum phosphorus is generally subnormal in cats with hyperparathyroidism,
which may explain the low incidence of renal compromise reported in these individuals.8
In addition, the serum chemistry profile may indicate liver disease (increased liver enzyme activities, hyperbilirubinemia)
or gastrointestinal tract dysfunction (hypoalbuminemia or hypocholesterolemia). Investigate any abnormalities.
5. Thyroxine (T
Measure serum T4 concentration in any patient more than 7 years old or in any cat showing signs of hyperthyroidism. A total T4 concentration is generally sufficient, but equivocal results can be investigated with free T4 measurement. Cats with hyperthyroidism often have increased PTH concentrations. This elevation may be an appropriate physiologic
response to increased calciuresis, but it can certainly confuse patient evaluation.9 Hypercalcemia (based on tCa concentration) was reported in two hyperthyroid cats, but no direct association was demonstrated
and no information was provided about reversibility.1