Calcium to phosphorous ratio (Ca/P) as helpful index to recognize primary hyperparathyroidism, but not primary hypoparathyroidism: a big-data approach.

Background Primary hyperparathyroidism (HyperPT) and primary hypoparathyroidism (HypoPT) are often underdiagnosed. Several strategies have been investigated in the past in order to identify diagnostic parameters, although the diagnosis of both HyperPT and HypoPT remains challenging so far, especially in asymptomatic patients. Calcium (Ca) and phosphorus (P) are inversely related together, thus the Ca/P ratio could be an useful tool to define these conditions. Recently, we proposed for the first time a cut-off of 3.5 for Ca/P ratio for the diagnosis of HyperPT. Aim To evaluate the diagnostic value of the Ca/P ratio for HyperPT and HypoPT through a big-data approach. Methodology A retrospective, observational, case-control study on big-data was carried out. All examinations of parathyroid hormone (PTH), Ca and P performed at the laboratory of Modena Hospital from 2010 to 2016 were consecutively included. We considered only patients between 18 and 90 years of age. Laboratory ranges of normality for both PTH and Ca were used to divide records in HyperPT, HypoPT and controls. Statistical analysis The diagnostic accuracy of Ca/P ratio was investigated using receiver operator characteristics (ROC) curves in order to define cut-off points, which show higher sensitivity and specificity for the identification of affected patients. Results 46 597 records were considered. 576 HyperPT (1.2%), 323 HypoPT (0.7%) and 45 698 controls (98.1%) were found. Ca/P ratio was significantly different among groups (P!0.001). In particular, Ca/P ratio was significantly higher in HyperPT than controls (P!0.001). For the diagnosis of HyperPT, the threshold of 3.17 for Ca/P ratio was obtained by means of the ROC curve analysis, with 85% of both sensitivity and specificity. HypoPT showed lower Ca/P ratio compared to controls (P!0.001), although no useful threshold for the diagnosis was found at ROC curve because of the low sensitivity. Conclusions We confirm the high sensitivity and specificity of Ca/P ratio for the diagnosis of HyperPT using the largest cohort of patients available so far in the literature. On the contrary, Ca/P ratio does not contribute to identify patients with HypoPT and further researches are needed to better describe this condition. In conclusion, Ca/P ratio is a simple and inexpensive diagnostic tool to recognize HyperPT.