Calcium/PTH Calculator

Total serum calcium is bound to albumin in a roughly predictable way, so a low albumin level (common in illness, malnutrition, or liver disease) can make total calcium read falsely low even when the physiologically active, unbound fraction is normal. This calculator applies the Payne correction formula -- named for Payne RB and colleagues, who derived it from 200 paired specimens published in the British Medical Journal in 1973 -- corrected calcium equals measured total calcium plus 0.8 times the difference between 4.0 g/dL and the patient's actual albumin, to adjust for that binding effect before classifying calcium status as low, normal, or high. It also reports a rough ionized calcium estimate at roughly half of total calcium, a convenience approximation only; a directly measured ionized calcium from a blood gas analyzer is materially more accurate and should be preferred when available or when the diagnosis is unclear.

From there, the calculator cross-references corrected calcium and PTH status to suggest a diagnostic pattern: high calcium with high PTH points toward primary hyperparathyroidism, high calcium with low PTH points toward a non-parathyroid cause of hypercalcemia (such as malignancy), low calcium with high PTH points toward secondary hyperparathyroidism (often driven by vitamin D deficiency or renal disease), and low calcium with low PTH points toward hypoparathyroidism. Any other combination -- most notably high calcium alongside a PTH that is merely normal-range rather than suppressed -- is reported as an indeterminate pattern rather than silently defaulting to "normal." That specific combination deserves particular attention: a healthy parathyroid axis suppresses PTH in response to hypercalcemia, so a mid-range PTH in a hypercalcemic patient is itself abnormal and is a classic, common presentation of primary hyperparathyroidism even though the PTH value alone doesn't clear this calculator's "high" threshold. Vitamin D status is reported separately using the deficiency/insufficiency/sufficiency/excess cutoffs from the Endocrine Society's 2011 vitamin D clinical practice guideline (other groups, such as IOM/NASEM, draw the sufficiency line differently) and does not feed into the diagnostic pattern classification, since vitamin D deficiency is more often a downstream cause to investigate once a secondary hyperparathyroidism pattern is suspected than a direct input to the calcium/PTH pattern itself.