Calculate the urine protein to creatinine ratio (UPCR) from a spot urine sample, convert it between mg/g, mg/mg and mg/mmol, and estimate 24-hour protein excretion.
Protein to Creatinine Ratio Formula
The ratio itself is a simple division of two values measured on the same spot urine sample.
When both values are in mg/dL, that division gives the ratio in mg of protein per mg of creatinine. Multiplying by 1000 puts it in the mg/g form most United States labs report.
The standard 24-hour protein estimate assumes an adult puts out about 1 gram of creatinine per day, so the mg/mg ratio is read directly as grams of protein per day.
The body size adjusted estimate replaces that 1 gram assumption with an expected creatinine excretion rate calculated from sex, age and weight.
- PCR = protein to creatinine ratio
- UP = urine protein concentration in the spot sample (mg/dL)
- UCr = urine creatinine concentration in the same sample (mg/dL)
- P24 = estimated 24-hour urine protein (g/day)
- eCER = expected 24-hour creatinine excretion (mg/day)
- W = body weight (kg)
- A = age (years)
- F = 1 for female, 0 for male
- P24adj = body size adjusted 24-hour urine protein (g/day)
The two calculator modes use these pieces differently. In the first mode you enter urine protein and urine creatinine and the calculator divides them, converting your entered units to mg/dL first, so g/L or mmol/L results from a non-US report still work. In the second mode you enter a ratio your lab already reported and the calculator only converts and interprets it, which is what you need when a report gives 40 mg/mmol and you want the mg/g equivalent.
The patient context selector changes nothing in the math. It only chooses which reference thresholds are applied to the result, because the number that counts as abnormal in pregnancy is not the number that counts as abnormal in a toddler. The optional body size adjustment adds the eCER step, which is worth using for a small older woman or a large young man, where the 1 gram per day assumption is furthest off.
Reference Thresholds and Expected Creatinine Output
The first table gives the cut points the calculator applies in each context, in all three reporting units.
| Context | mg/g | mg/mg | mg/mmol | Meaning |
|---|---|---|---|---|
| Adult | Under 150 | Under 0.15 | Under 17 | Normal to mildly increased (A1) |
| Adult | 150 to 500 | 0.15 to 0.5 | 17 to 57 | Moderately increased (A2) |
| Adult | Over 500 | Over 0.5 | Over 57 | Severely increased (A3) |
| Adult | 3500 or more | 3.5 or more | 396 or more | Nephrotic range |
| Pregnancy | 300 or more | 0.3 or more | 34 or more | Significant proteinuria in a preeclampsia workup |
| Child over 2 years | Under 200 | Under 0.2 | Under 23 | Normal |
| Infant 6 to 24 months | Under 500 | Under 0.5 | Under 57 | Normal |
The second table is the part most ratio pages leave out. The “ratio equals grams per day” shortcut only holds if you actually excrete 1 gram of creatinine a day. These are expected daily creatinine outputs from the CKD-EPI creatinine excretion equation, with the multiplier you would apply to a mg/mg ratio to get a size adjusted daily protein figure.
| Sex | Age | Weight | Expected creatinine (mg/day) | Multiplier |
|---|---|---|---|---|
| Male | 30 | 70 kg | 1570 | 1.57 |
| Male | 30 | 90 kg | 1820 | 1.82 |
| Male | 60 | 70 kg | 1384 | 1.38 |
| Male | 80 | 70 kg | 1260 | 1.26 |
| Female | 30 | 50 kg | 940 | 0.94 |
| Female | 30 | 70 kg | 1190 | 1.19 |
| Female | 60 | 70 kg | 1005 | 1.00 |
| Female | 80 | 70 kg | 881 | 0.88 |
Read across the table and the size effect is obvious. The same 0.5 mg/mg ratio predicts about 0.91 g/day of protein loss in a 90 kg 30 year old man and about 0.44 g/day in an 80 year old 70 kg woman. The ratio has not changed; the amount of creatinine it is being compared against has.
Example Problems
Example 1. A spot urine comes back with protein 45 mg/dL and creatinine 120 mg/dL in a 62 year old woman who weighs 58 kg.
Divide: 45 / 120 = 0.375 mg/mg. In mg/g that is 375, and in mg/mmol it is 375 / 8.84 = 42.4. A ratio of 375 mg/g falls in the KDIGO A2 band, moderately increased protein. The plain 24-hour estimate is 0.375 g/day. Her expected creatinine excretion is 879.89 + 12.51(58) – 6.19(62) – 379.42 = 842 mg/day, so the size adjusted estimate is 375 x 842 / 1000000 = 0.32 g/day. The unadjusted number overstates her daily protein loss by about 19 percent because she excretes well under 1 gram of creatinine a day.
Example 2. A patient at 34 weeks of pregnancy with new hypertension has a spot urine protein of 30 mg/dL and creatinine of 85 mg/dL.
Divide: 30 / 85 = 0.353 mg/mg, which is 353 mg/g or 39.9 mg/mmol. In the pregnancy context that sits above the 0.3 mg/mg cut point, so it meets the usual threshold for significant proteinuria in a preeclampsia workup. The body size adjustment is not applied here, since the pregnancy threshold is defined on the raw ratio.
Frequently Asked Questions
My lab reported the ratio in mg/mmol. How do I compare it to a mg/g number?
Multiply mg/mmol by 8.84 to get mg/g, or divide mg/g by 8.84 to go the other way. The factor comes from the molar mass of creatinine, 113.12 g/mol: one gram of creatinine is 8.84 mmol. So 30 mg/mmol is 265 mg/g, and the 300 mg/g pregnancy threshold is about 34 mg/mmol. Use the second calculator mode to convert a reported ratio without re-entering the raw protein and creatinine values.
Does the ratio really replace a 24-hour urine collection?
For most monitoring purposes yes, and it avoids the biggest problem with timed collections, which is that people miss voids or collect for the wrong length of time. The ratio also cancels out how dilute the sample is, because protein and creatinine are concentrated or diluted together. It is less reliable at the extremes: someone with very low muscle mass excretes little creatinine and their ratio will overstate protein loss, while a heavily muscled person’s ratio understates it. That is exactly what the body size adjustment in the calculator is for. A first morning or second morning void is preferred over a random daytime sample because it removes the effect of upright posture.
What can push the ratio up when the kidneys are fine?
Fever, a urinary tract infection, heavy exercise in the hours before the sample, dehydration, menstrual blood contamination and orthostatic proteinuria in adolescents can all raise a single result. Orthostatic proteinuria in particular is confirmed by comparing a first morning sample, which should be normal, against a daytime sample. A single elevated ratio is a reason to repeat the test, not a diagnosis. Persistent elevation over three months is what defines chronic kidney disease along with the eGFR.