Reliability of calcium-phosphorus (Ca/P) ratio as a new, accurate and inexpensive tool in the diagnosis of some Ca-P disorders.

PURPOSE: The serum calcium/phosphorus (Ca/P) ratio is an accurate tool to differentiate patients with primary hyperparathyroidism (PHPT) from healthy subjects. However, other disorders of the Ca-P metabolism might impair the Ca/P ratio, such as hypophosphatemia (HypoP) not PHPT related. The aim of this study is to examine the diagnostic value of Ca/P ratio in the diagnosis of PHPT and HypoP not PHPT related. METHODS: Single-center, retrospective, case-control study, including 150 patients with PHPT and 306 patients with HypoP, compared with 150 controls. HypoP patients were enrolled among HIV-infected patients by selecting those with Fanconi-like syndrome due to antiretroviral treatment. Parameters which were measured were serum Ca, P, parathyroid hormone (PTH), 25-OH vitamin D, albumin and creatinine). RESULTS: The Ca/P ratio was significantly higher in PHPT and HypoP patients, compared to controls (p < 0.0001). At receiver operator characteristic (ROC) curve analysis, the cut-off of 3.56 (2.75 SI) for Ca/P ratio was able to identify patients with PHPT and HypoP (sensitivity 95%; specificity 93%). Among patients with Ca/P ratio above 3.56, the thresholds of 10.3 mg/dL (2.6 mmol/L) for serum Ca (sensitivity 93%; specificity 98%) and 80.5 pg/mL for PTH (sensitivity 91%; specificity 91%) were defined for the specific diagnosis of PHPT. CONCLUSIONS: The Ca/P ratio above 3.56 (2.75 SI) is a highly accurate tool to identify PHPT and HypoP not PHPT-related patients. Thanks to its simplicity, this index can be proposed as a screening and first-line examination in the diagnostic work-up when a disorder of Ca-P metabolism is suspected or should be ruled out.