The Utility of the Cl:PO4 Ratio in Patients With Variant Versions of Primary Hyperparathyroidism.

Objective: To determine the significance and impact of additional chloride testing as part of a diagnostic laboratory test battery for borderline primary hyperparathyroidism (pHPT).
Study Design: Retrospective database review of parathyroidectomy patients.
Setting: A tertiary care, academic health sciences center.
Subjects and Methods: Patients referred to a head and neck endocrine clinic for evaluation and treatment for pHPT.
Results: After exclusions, there were a total of 226 patients who underwent parathyroidectomy for primary hyperparathyroidism with the requisite preoperative and postoperative labs. Seventy-seven additional patients who had a thyroid operation for a nonmalignant cause were included as controls. Of the 303 total patients, 166 had normal calcium levels (<10.4 mg/dL), and 54 (32.5%) also exhibited hyperchloremia (>106 mmol/L). Of the 47 patients with normal calcium and parathyroid hormone (PTH) levels (<88 pg/mL), 6 (12.8%) had hyperchloremia, and of the 118 patients with normocalcemic pHPT, 48 (40.7%) were hyperchloremic. The area under the curve for the Cl:PO4 was 0.712. When using a cutoff of 33, the reported sensitivity and specificity of the curve were 58.4% and 28.6%, respectively.
Conclusion: The Cl:PO4 ratio was a moderately sensitive test for the diagnosis of the borderline variants of primary hyperparathyroidism. The Ca:PO4 ratio was superior to the Cl:PO4 ratio. Our data also showed the superiority of preoperative calcium and ionized calcium over PTH when predicting the presence of pHPT.