Spot Urine Protein: urine protein-to-creatinine ratio, calculation and interpretation
What the protein-to-creatinine ratio is and when it is used
The urine protein-to-creatinine ratio (UPCR, also abbreviated PCR) is a way of quantifying protein loss in the urine using a single sample, without a 24-hour collection. Creatinine serves as the reference because its excretion over the day is relatively constant, which corrects for the effect of more concentrated or more dilute urine.
The method is based on the study by Ginsberg and colleagues, published in the New England Journal of Medicine in 1983. In 46 samples, the ratio in a spot urine correlated excellently with the protein in 24-hour urine. The best correlation appeared in samples collected after the first morning void and before bedtime.
The KDIGO 2024 guideline places the UPCR as the first option when total urine protein is measured, and the albumin-to-creatinine ratio as the first option for the initial assessment of albuminuria. The guideline prefers a first morning, midstream urine. To compare with the albumin measurement, see the albumin-to-creatinine ratio calculator.
How to calculate urine protein in the calculator: inputs and units
The calculator asks for two values from the same urine sample:
- Urine protein, in mg/dL, from 0 to 5,000.
- Urine creatinine, in mg/dL, greater than zero and up to 1,000.
Values outside these ranges produce an error message with the most likely unit conversion.
The tool divides protein by creatinine and displays:
- Protein-to-creatinine ratio (UPCR), in g/g (numerically equal to mg/mg), with two decimal places;
- UPCR in mg/g, as a whole number, the unit of the KDIGO tables;
- Estimated 24-hour urine protein, in g/24 h, approximate;
- KDIGO category A1, A2 or A3, with a nephrotic-range warning when the UPCR exceeds 3.5 g/g.
The critical point is the unit. Both measurements must be in mg/dL. If protein comes in mg/L, divide by 10 before entering it. If urine creatinine comes in mmol/L, convert to mg/dL by multiplying by 11.31 (1 mg/dL = 88.4 µmol/L, the KDIGO 2024 factor). Mixing mg/L with mg/dL multiplies or divides the result by 10.
Protein-to-creatinine ratio formula and why it estimates g/24 h
The protein-to-creatinine ratio is a direct division between two concentrations in the same urine.
UPCR (mg/mg = g/g) = Urine protein (mg/dL) ÷ Urine creatinine (mg/dL)
Estimated urine protein (g/24 h) ≈ UPCR (g/g)
- Urine protein: total protein measured in the spot sample.
- Urine creatinine: creatinine from the same sample.
- UPCR: grams of protein per gram of creatinine. Multiplied by 1,000, it gives mg/g.
The equivalence with grams per 24 hours rests on an assumption: an average adult excretes about 1 g of creatinine per day. If actual excretion is higher, the UPCR underestimates daily proteinuria; if it is lower, it overestimates it. Ginsberg and colleagues emphasized that the ratio should be interpreted taking differences in creatinine excretion into account. KDIGO 2024 reinforces the point: women and people of low body weight tend to have a higher UPCR, and men and people of high body weight a lower UPCR, relative to timed excretion.
The calculator makes no adjustment for sex or muscle mass.
Worked example of spot urine protein
Hypothetical case: urine protein of 45 mg/dL and urine creatinine of 90 mg/dL, in the same sample.
- Divide protein by creatinine: 45 ÷ 90 = 0.5.
- UPCR: 0.50 g/g, or 500 mg/g.
- Estimated urine protein: 0.50 g/24 h, approximately.
- Category: 500 mg/g is the upper limit of the 150 to 500 range, therefore A2, moderately increased.
The calculator returns 0.50 g/g, 500 mg/g, 0.50 g/24 h and category A2 for these data.
Consider the effect of a unit error. If a protein of 450 mg/L were entered without dividing by 10, the calculation would give 450 ÷ 90 = 5.00 g/g (5,000 mg/g): category A3 and a nephrotic-range warning. The same test, with the unit mixed up, would change category.
According to KDIGO 2024, a UPCR of 500 mg/g corresponds approximately to 50 mg/mmol and is equivalent, in order of magnitude, to an albumin-to-creatinine ratio of 300 mg/g, the threshold of severely increased albuminuria.
How to understand the result: KDIGO categories A1, A2 and A3
The calculator classifies the UPCR by the KDIGO proteinuria categories, published in the 2012 guideline and maintained in the 2024 one. The limits in mg/g assume creatinine excretion of about 1 g per day.
| KDIGO category | UPCR (mg/g) | Description |
|---|---|---|
| A1 | below 150 | Normal to mildly increased |
| A2 | 150 to 500 | Moderately increased |
| A3 | above 500 | Severely increased |
The category is applied to the whole value in mg/g shown on screen, and the limits of 150 and 500 belong to category A2. When the UPCR exceeds 3.5 g/g, the tool adds a warning of nephrotic-range proteinuria, a threshold described by Ginsberg and colleagues (1983). Nephrotic range is not synonymous with nephrotic syndrome, which also involves other findings.
The category applies to persistent proteinuria. KDIGO defines chronic kidney disease by abnormalities present for at least three months, and an abnormal result should be confirmed in a new sample. Interpretation is the responsibility of the professional.
Common errors, sources of variation and checklist
The UPCR is practical, but several factors change the result without any real change in kidney function. KDIGO 2024 lists the main ones.
- Falsely increase the ratio: hematuria, menstruation, exercise, symptomatic urinary tract infection, female sex and low body weight (less urine creatinine), acute kidney injury and a low-protein diet.
- Falsely decrease the ratio: male sex, high body weight, a high-protein diet and exercise that increases creatinine excretion.
Common procedural errors:
- mixing mg/L and mg/dL between protein and creatinine;
- using protein and creatinine from different samples;
- collecting after intense exercise;
- treating a single abnormal UPCR as definitive, without confirmation.
Checklist:
- Are protein and creatinine in mg/dL and from the same sample?
- Is the sample, preferably, a first morning, midstream urine?
- Is there hematuria, menstruation, infection or recent exercise?
- Is the patient's muscle mass far from average?
- Was the abnormal result confirmed in a new sample?
Limitations and comparison with 24-hour urine and albuminuria
The main limitation of the UPCR is that it depends on an assumed creatinine excretion. In people with a lot or very little muscle mass, the g/24 h estimate departs from reality. There are also analytical limits: according to KDIGO 2024, total urine protein measurement has no reference method or standardized material, and results can vary by more than 40% between laboratories, depending on the methodology.
| Method | Collection | Note |
|---|---|---|
| Protein-to-creatinine ratio | Spot sample | Practical; depends on creatinine excretion |
| 24-hour urine protein | 24-hour urine | Measures daily excretion; subject to incomplete collection |
| Albumin-to-creatinine ratio | Spot sample | KDIGO 2024 first option for albuminuria |
| Dipstick | Spot sample | Semiquantitative; a positive result should be quantified |
When the spot-sample estimate seems inconsistent, 24-hour urine protein measures daily excretion directly. Proteinuria is read together with filtration, such as the eGFR by CKD-EPI 2021, because KDIGO classifies risk by combining the two.
Frequently asked questions about spot urine protein
Does the protein-to-creatinine ratio replace 24-hour urine?
In most situations, yes. Ginsberg and colleagues (1983) showed good correlation between the ratio in a spot sample and 24-hour protein. At extremes of muscle mass, the estimate loses accuracy and a 24-hour collection may be necessary.
What is the normal protein-to-creatinine ratio?
According to KDIGO, a UPCR below 150 mg/g (0.15 g/g) corresponds to category A1, normal to mildly increased. The study by Ginsberg and colleagues (1983) considered a ratio below 0.2 mg/mg normal.
Are mg/mg, g/g and g/24 h the same thing?
Mg/mg and g/g are numerically equal. The value in g/24 h is an approximation, valid when the person excretes about 1 g of creatinine per day. For mg/g, multiply the result by 1,000.
My urine protein is in mg/L. How do I use the calculator?
Divide by 10 to obtain mg/dL before entering it. Creatinine must also be in mg/dL. Mixing the units multiplies or divides the result by 10.
Which urine sample is best?
KDIGO 2024 prefers a first morning, midstream urine for adults and children. In the 1983 study, the best correlation appeared in samples collected after the first morning void and before bedtime.
References
- Ginsberg JM, Chang BS, Matarese RA, Garella S. Use of single voided urine samples to estimate quantitative proteinuria. N Engl J Med. 1983;309(25):1543-6. View
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117-S314. View
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013;3(1):1-150. View
Updated . A support tool for educational purposes; it does not replace clinical assessment.