Understanding the Hammersmith Clearance Ratio
The Hammersmith urine calcium creatinine clearance ratio compares renal calcium clearance with creatinine clearance. It is often used during the review of raised blood calcium. The aim is to separate patterns that may fit familial hypocalciuric hypercalcaemia from patterns that may fit primary hyperparathyroidism. It does not make a diagnosis alone.
Why The Ratio Matters
Serum calcium can look similar in different conditions. Urine calcium handling may provide another clue. Familial hypocalciuric hypercalcaemia usually has relatively low urinary calcium. Primary hyperparathyroidism more often has higher urinary calcium, although overlap can happen. Renal function, vitamin D status, calcium intake, medicines, and collection quality can all change the result.
Data Needed
The calculation needs serum calcium, serum creatinine, urine calcium, and urine creatinine. The Hammersmith layout commonly uses serum creatinine in micromoles per litre. The calculator accepts several units and converts them before the equation runs. This helps reduce unit mistakes. Use values from samples taken close together whenever possible.
Interpreting Outputs Carefully
A clearance ratio below 0.01 may support possible familial hypocalciuric hypercalcaemia. A value above 0.02 usually argues against it. Results between 0.01 and 0.02 form a grey zone. The spot calcium creatinine ratio is shown separately. It is useful for other urine reviews, but it is not the same as the clearance ratio.
Practical Checks
Check that serum and urine units match the selected options. Review outliers before using the report. Very low urine creatinine may reflect a dilute sample. Low vitamin D, low calcium intake, thiazide use, lithium use, and kidney disease may change urinary calcium. Repeat testing or genetic review may be needed when the clinical picture and ratio disagree.
Using The Result
Treat the result as a decision support value. Share it with a qualified clinician and the laboratory team. Keep the source values with the report. The CSV and PDF downloads make that easier. They also help document assumptions, units, corrected calcium, and calculation notes for future review.
Before exporting, compare the ratio with symptoms, calcium history, family history, and parathyroid hormone. The best interpretation uses the whole record. Numbers are helpful, yet context protects patients from rushed conclusions and missed repeat testing when needed later.