Serum ionized calcium in dogs with chronic renal failure and metabolic acidosis

Background: Chronic renal failure (CRF) is a common disease in dogs, and many metabolic disorders can be observed, including metabolic acidosis and calcium and phosphorus disturbances. Acidosis may change the ionized calcium O-Ca) fraction, usually increasing its concentration. Objective: In this study we evaluated the influence of acidosis on the serum concentration of i-Ca in dogs with CRF and metabolic acidosis. Methods: Dogs were studied in 2 groups: group I (control group = 40 clinically normal dogs) and group 11 (25 dogs with CRF and metabolic acidosis). Serum i-Ca was measured by an ion-selective electrode method; other biochemical analytes were measured using routine methods. Results: The i-Ca concentration was significantly lower in dogs in group 11 than in group 1; 56% of the dogs in group 11 were hypocalcemic. Hypocalcemia was observed in only 8% of dogs in group 11 when based on total calcium (t-Ca) concentration. No correlation between pH and i-Ca concentration was observed. A slight but significant correlation was detected between i-Ca and serum phosphorus concentration (r = -.284; P =.022), as well as between serum t-Ca and i-Ca concentration (r =.497; P <.0001). Conclusion: The i-Ca concentration in dogs with CRF and metabolic acidosis varied widely from that of t-Ca, showing the importance of determining the biologically active form of calcium. Metabolic acidosis did not influence the increase in i-Ca concentration, so other factors besides acidosis in CRF might alter the i-Ca fraction, such as hyperphosphatemia and other compounds that may form complexes with calcium.