Do I need to avoid diuretics, such as furosemide (Lasix) and hydrochlorothiazide (HCTZ), before a 24-hour urine calcium test?

Diuretic Use Before 24-Hour Urine Calcium Testing

Yes, you should avoid diuretics before a 24-hour urine calcium test, as both loop diuretics (furosemide) and thiazide diuretics (hydrochlorothiazide) significantly alter urinary calcium excretion and will produce inaccurate results that do not reflect your baseline calcium metabolism.

Why Diuretics Must Be Avoided

Loop Diuretics (Furosemide/Lasix)

• Furosemide dramatically increases urinary calcium excretion, with studies showing 3-4 fold increases in calcium loss during both acute and chronic administration 1
• A single dose of furosemide (15 mg/kg) enhanced urinary calcium excretion within 4 hours, and chronic twice-daily administration sustained this calciuretic effect 1
• In human studies, furosemide caused significant calciuresis of 132.0 ± 15.2 mg over 6 hours despite elevating serum calcium concentrations 2
• This artificially elevated urinary calcium would falsely suggest hypercalciuria when none may exist 1, 2

Thiazide Diuretics (Hydrochlorothiazide/HCTZ)

• Thiazides have the opposite effect—they reduce urinary calcium excretion, which is precisely why they are used therapeutically in kidney stone patients with hypercalciuria 3
• The American Urological Association recommends thiazide diuretics specifically to lower urine calcium in patients with high urinary calcium and recurrent calcium stones 3
• With prolonged hydrochlorothiazide use, urate and calcium excretion both fell significantly 4
• This artificially suppressed urinary calcium would mask true hypercalciuria 4

Clinical Implications

Test Accuracy

• The 24-hour urine calcium collection is the gold standard for diagnosing hypercalciuria, as spot urine calcium-to-creatinine ratios are not interchangeable with 24-hour collections 5
• Fasting spot urine underestimated 24-hour calcium with 0% sensitivity for detecting hypercalciuria, while postprandial specimens had only 77% sensitivity and 61% specificity 5
• Any medication that alters calcium handling will compromise the diagnostic accuracy of this already-challenging test 5

Medication Timing

• If medically safe, discontinue diuretics for an appropriate washout period before testing 6
• The duration of washout depends on the diuretic's half-life and duration of action—furosemide has a 6-8 hour duration of action, while hydrochlorothiazide acts for 6-12 hours 3
• For chronic users, the compensatory physiologic changes may take several days to normalize after discontinuation 1, 4
• Consult with the prescribing physician before stopping any diuretic, particularly in patients with heart failure, hypertension, or volume overload where abrupt cessation could be harmful 3, 6

Common Pitfalls

• Do not assume that because a patient has been on stable diuretic therapy, the test results will be "consistently altered"—the magnitude of calcium excretion changes varies and cannot be reliably corrected mathematically 1, 2, 4
• Avoid testing during acute illness or volume depletion, as these states independently affect calcium handling 6
• Ensure the patient is not taking other medications that affect calcium metabolism during the collection period 6