TABLE 1.

Definitions of diagnoses for ionized hypercalcemia in cats

Diagnosis	Definition
Acute kidney injury (AKI)	At least 1 iCa >1.41 mmol/L after acute development of azotemia (AKI) or acute worsening of chronic azotemia (acute‐on‐chronic kidney disease) Unless: Previous ionized hypercalcemia had been documented before the occurrence of AKI Hypercalcemia persisted at the same magnitude despite improvement in azotemia Renal neoplasia was identified as the cause of the AKI
Chronic kidney disease (CKD)/renal diet‐associated	At least 1 iCa >1.41 mmol/L after feeding a diet formulated for feline CKD, or hypercalcemia that resolved after cessation of this diet Cats with CKD with iCa >1.41 mmol/L and no other comorbidities identified
Granulomatous	At least 1 iCa >1.41 mmol/L AND Granulomatous inflammation identified on cytology or histopathology OR Granulomas reported on ophthalmic examination
Iatrogenic	At least 1 iCa >1.41 mmol/L that occurred only after administration of oral or intravenous calcium supplementation, or veterinary prescribed vitamin D supplementation Cases with acute kidney injury (AKI) provided iCa was normal or low before administration of calcium
Idiopathic (IHC)	At least 1 iCa >1.41 mmol/L AND No evidence of another disease process after full diagnostic work up Hypercalcemia did not resolve without specific medication to decrease iCa The presence of a normal plasma PTH concentration did not exclude a case from this diagnosis given the variability of assays available if the patient did not develop further clinical signs indicative of another pathological process in the following 3 months
Juvenile	Under 1 year of age iCa >1.41 mmol/L, but ≤1.6 mmol/L No disease process identified that has been associated with ionized hypercalcemia
Malignancy‐associated	At least 1 iCa >1.41 mmol/L AND Diagnosis of a neoplasm associated with hypercalcemia, either by osteolysis (if evident on imaging), primary or metastatic bone neoplasia or parathyroid hormone‐related peptide (PTHrP) (para‐neoplastic) production If a case was treated for presumptive neoplasia without further investigations to confirm PTHrP production or T‐cell lymphoma, it was included if no other cause for the hypercalcemia was identified and hypercalcemia resolved with tumor remission Patients with functional parathyroid neoplasia were not included in this group
Primary hyperparathyroidism	iCa >1.41 mmol/L AND Plasma parathyroid hormone (PTH) concentration above, or in the upper half of the laboratory reference interval Concurrent serum phosphate concentration below or in the bottom half of the reference interval AND/OR Parathyroid nodule on cervical imaging normo‐ or hypo‐calcemic at reassessment postparathyroidectomy Histopathological evidence of a parathyroid adenoma, adenocarcinoma or hyperplasia
Toxicity	At least 1 iCa >1.41 mmol/L AND Increased serum calcitriol or 25‐hydroxyvitamin D concentrations AND/OR With a history of vitamin D exposure Marked hypercalcemia that resolved rapidly with supportive care and where full assessment or necropsy failed to identify another cause
Transient	At least 1 iCa >1.41 mmol/L which resolved without treatment recognized to decrease iCa No disease process identified that has been associated with ionized hypercalcemia
Undetermined	At least 1 iCa >1.41 mmol/L Diagnosis could not be assigned based on the available information OR >1 diagnosis present that is known to cause ionized hypercalcemia and the primary condition could not be determined

Abbreviations: AKI, acute kidney injury; CKD, chronic kidney disease; iCa, ionized calcium; PTH, parathyroid hormone.