Urine Citrate Unit Converter
Urine Citrate Unit Converter
Citrate is the inhibitor, and hypocitraturia is the finding — present in about a third of stone formers. Convert mg/24 h, µg/24 h, µmol/24 h and nmol/24 h.
Urine Citrate converter
Mass ⇄ molar, per 24 hA 24-hour urinary citrate of 320 mg, the usual hypocitraturia threshold
Formula and conversion factors
nmol/24 h = µg/24 h × 5.205
mmol/24 h = mg/24 h × 0.005205
mg/24 h = mmol/24 h ÷ 0.005205
- 192.123
- the molar mass of citric acid, C6H8O7 — the mass laboratories convert with, and the reason ARUP and Labcorp both call the test citric acid
- 189.099
- the molar mass of the citrate trianion, the species actually in urine; 1.6% lower, and not what the published factors use
- 320 mg = 1.67 mmol
- the hypocitraturia threshold in both unit systems, and a check on the factor
Worked example
A 24-hour urinary citrate of 320 mg, the usual hypocitraturia threshold
320 × 5.205 = 1,666 µmol/24 h
= 1.67 mmol/24 h, the figure the stone literature quotes for the same threshold
Exactly on ARUP's and Labcorp's lower limit of 320 mg/day, comfortably above Mayo's lower limit at age 20 (150 mg/day) and below it at age 60 (434 mg/day)
One result, three verdicts, depending on whose interval and which age it is read against
24-hour urine citrate: the published intervals
| Source | Interval | Unit |
|---|---|---|
| ARUP, 18 years and over | 320 – 1,240 | mg/day |
| Labcorp 016865, adult | 320 – 1,240 | mg/24 h |
| Mayo CITR, age 20 | 150 – 1,191 | mg/24 h |
| Mayo CITR, age 40 | rises with age | mg/24 h |
| Mayo CITR, age 60 | 434 – 1,191 | mg/24 h |
| Hypocitraturia, conventional definition | < 320 | mg/day |
| Suggested target for a recurrent stone former | ≥ 640 | mg/day |
| The same two figures in SI | 1.67 and 3.33 | mmol/day |
Which molar mass, and what it changes
| Basis | Molar mass | mg/24 h to µmol/24 h |
|---|---|---|
| Citric acid, C6H8O7 — used here | 192.123 | 5.205 |
| Citrate trianion, C6H5O7 3− | 189.099 | 5.2882 |
| Difference | 1.6% | Three times the 0.5% a consistency check tolerates |
The inhibitor, and why its absence is the finding
Citrate chelates calcium in the tubular lumen and in the final urine, so calcium that is bound to citrate is calcium that cannot precipitate with oxalate or phosphate. It also inhibits crystal nucleation and growth directly. The consequence is that citrate is read the opposite way round from almost everything else on a stone panel: the abnormality is a low value, not a high one, and a citrate near the bottom of the reference interval in a recurrent stone former is a therapeutic target rather than a reassurance.
Hypocitraturia is conventionally defined as below 320 mg/day, equivalently 1.67 mmol/day. It is found in something like 30% of all stone formers — estimates run from 10% to 60% depending on the definition and the stone type — and is the sole identifiable abnormality in about one in ten calcium stone formers. Several authors argue the reference-derived threshold is too permissive and that 640 mg/day is the figure to aim for in someone who keeps making stones, which is a statement about risk rather than about normality.
Urinary citrate is governed by acid-base balance. The proximal tubule reabsorbs filtered citrate, and acidosis increases that reabsorption, so anything that acidifies lowers urinary citrate: distal renal tubular acidosis (where hypocitraturia, alkaline urine and calcium phosphate stones travel together), chronic diarrhoea and bowel disease, a heavy animal protein load, and hypokalaemia from a thiazide. The therapy is the mirror image — potassium citrate, where roughly 30 mEq a day raises urinary citrate by about 200 mg/24 h. Read citrate alongside urine calcium, urine oxalate and urine uric acid rather than alone; the 24-hour urine stone-risk interpreter does that against guideline limits.
Two practical points about the units. ARUP and Labcorp both label the test citric acid and report mg/day; Mayo reports mg/24 h; European and most other SI laboratories report mmol/day. And the collection is part of the result: citrate is an amount per 24 hours, so an incomplete bottle manufactures hypocitraturia out of nothing. Check the creatinine on the same collection with the collection adequacy calculator.
Frequently asked questions
How do I convert urine citrate from mg/24 h to mmol/24 h?
Multiply by 0.005205, one divided by the molar mass of citric acid, 192.123. So 320 mg/day is 1.67 mmol/day and 640 mg/day is 3.33 mmol/day. For micromoles multiply the milligrams by 5.205.
What level of urinary citrate counts as hypocitraturia?
Below 320 mg/day, or 1.67 mmol/day, is the conventional definition and is also ARUP’s and Labcorp’s lower reference limit. Mayo’s lower limit is age-dependent and ranges from about 150 mg/24 h at age 20 to 434 mg/24 h at age 60, so the same result can be normal at one laboratory and low at another.
Why do ARUP and Labcorp call it citric acid rather than citrate?
Because the assay is calibrated and converted as citric acid, molar mass 192.123, even though urine contains the ionised forms. The naming is a clue to the arithmetic: using the trianion’s 189.099 instead would shift every converted value by 1.6%.
Is a high urine citrate a problem?
No. High citrate is protective against calcium stones, and a raised value usually just means alkali — citrate supplementation, a vegetarian diet or an alkaline load. There is no clinical syndrome of hypercitraturia.
Can citrate be measured on a spot urine?
It can be reported as a citrate-to-creatinine ratio, which is how paediatric specimens are usually handled, but the stone-risk thresholds are daily excretions. A spot concentration cannot be converted into one without the collection volume.
Related calculators
References
- ARUP Laboratories. Citric acid, urine — per 24 h. Laboratory Test Directory 0020852 (component 0020242); 2026.
- Labcorp. Citric acid (citrate), 24-hour urine — test 016865, reference interval. Test menu; 2026.
- Mayo Clinic Laboratories. Citrate excretion, 24 hour, urine (CITR) — age-specific reference values. Test catalogue, test ID 606710; 2026.
- Hypocitraturia. In: StatPearls. Treasure Island (FL): StatPearls Publishing; point-of-care article 120007, accessed October 2026.
- PubChem. Citric acid, CID 311. National Library of Medicine; accessed October 2026.
- Human Metabolome Database. Citric acid, HMDB0000094 — average mass 192.1235.
- GlobalRPh. Conventional units to international units — citrate mg/dL x 52.05 to umol/L; oxalate mg/L x 11.1 to umol/L. Accessed October 2026.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/