Serum calcium to phosphorous ratio (Ca/P) as a simple, inexpensive screening tool in the diagnosis of primary hyperparathyroidism (PHPT).

Background The diagnosis of primary hyperparathyroidism (PHPT) is challenging and is based on serum calcium (Ca) and parathyroid hormone (PTH). As serum Ca and phosphorous (P) are inversely related in PHPT, the Ca/P ratio might be considered a good candidate tool in the diagnosis of PHPT. The aim of this study is to investigate the diagnostic value of the Ca/P ratio in the diagnosis of PHPT. Methods For this single-centre, case-controlled, retrospective study we collected data from review charts of 97 patients with documented PHPT (69 females; 28 males) and compared them with those of 96 controls (C) (44 females; 52 males). The retrospective nature of the study allows obtaining for certain the diagnosis of PHPT. Main outcome measures were: serum PTH, 25-OH Vitamin D, Ca, P, albumin, and creatinine. Results Among PHPT patients, 35 (36.1%) had normocalcemic PHPT (NCHPT). Ca was significantly higher in PHPT (median: 11; min-max: 9.4-15.5) than C (9.4; 8.3-10.2) (p<0.0001). P was significantly lower in PHPT (2.4; 1.4-3.9) than in C (3.5; 2.1-4.5) (p<0.0001). PTH was significantly higher in PHPT (135.2; 57.6-1748) than in C (32.1; 14-80.7) (p<0.0001). Ca/P ratio was significantly higher in PHPT (4.6; 2.7-8.8) than in C (2.7; 2.0-4.6) (p<0.0001). ROC curves analyses identified a of 3.5 for Ca/P ratio with a sensitivity and specificity of 86% and 87%, respectively (p<0.0001). Conclusions Ca/P ratio is a valuable highly sensitive, highly specific tool for the diagnosis of PHPT. Besides, Ca/P has the best diagnostic value in identifying normocalcemic PHPT (NCPHPT). Considering that Ca/P is simple to obtain, easily accessible in every clinical and laboratory setting worldwide, and inexpensive even when used in large sample size of patients, this diagnostic tool could be useful for screening PHPT, especially in patients accessing emergency rooms or in the general practitioner setting. This biochemical index has the advantage to be universally used in all clinical settings for both the diagnosis and the screening of PHPT thanks to its low costs and worldwide availability in any laboratory setting (even in developing countries).