Why does a spot ratio estimate 24-hour excretion?

Creatinine is excreted into the urine at a fairly constant rate of about 10 mmol per day, so dividing protein or albumin by creatinine cancels out how dilute or concentrated the sample is. The resulting ratio tracks the daily excretion closely enough to replace most timed collections.

How do PCR and ACR differ?

PCR measures total protein and is useful for detecting and following heavier proteinuria such as glomerular disease. ACR measures albumin specifically and is more sensitive at low levels, which is why it is the preferred test for screening diabetic and chronic kidney disease patients.

What are the KDIGO albuminuria categories?

On ACR, A1 is below 3 mg/mmol and is normal to mildly increased, A2 is 3 to 30 mg/mmol and moderately increased, and A3 is above 30 mg/mmol and severely increased. These categories combine with the eGFR stage to grade chronic kidney disease risk.

How do I convert from the older mg/g units?

Some labs report in milligrams per gram of creatinine rather than milligrams per millimole. Divide a mg/g value by about 8.84 to convert it to mg/mmol, which is the unit this calculator and most international guidelines use.

When is the spot ratio unreliable?

Very high or very low muscle mass changes creatinine excretion away from the 10 mmol assumption, and rapidly changing kidney function, heavy exercise or a high-protein meal can distort a single sample. An early-morning specimen and, where it matters, a confirmatory timed collection improve accuracy.

Protein:Creatinine Ratio & Albumin:Creatinine Ratio Calculator

Quantifying how much protein a patient is leaking into the urine once meant an awkward, patient-unfriendly, and often inaccurate 24-hour urine collection. A spot urine ratio gives essentially the same clinical information from a single sample taken at any time of day, and this calculator turns the two raw lab numbers into a ratio, a daily excretion estimate, and the appropriate guideline category.

Why the spot ratio works

Creatinine is a waste product of muscle metabolism excreted into the urine at a relatively constant rate — approximately 10 mmol per day in adults of average muscle mass. Because the rate is constant, dividing the protein or albumin concentration by the creatinine concentration in the same spot sample cancels out the effect of urine dilution or concentration. A dilute urine sample has low protein and low creatinine; a concentrated one has high protein and high creatinine. The ratio is similar in both cases, making it a reliable surrogate for the 24-hour excretion.

How the calculation works

PCR (mg/mmol) = urine protein (mg/L)  ÷ urine creatinine (mmol/L)
ACR (mg/mmol) = urine albumin (mg/L)  ÷ urine creatinine (mmol/L)

With ~10 mmol of creatinine excreted daily, the 24-hour estimates follow directly:

PCR × 10 ≈ grams of total protein per day (a PCR of 100 mg/mmol ≈ 1 g/day)
ACR × 10 ≈ milligrams of albumin per day (an ACR of 30 mg/mmol ≈ 300 mg/day)

The tool maps the ACR onto the KDIGO albuminuria categories and the PCR onto common proteinuria severity bands.

KDIGO albuminuria categories (ACR)

The KDIGO 2012 classification is the current international standard for staging chronic kidney disease (CKD) risk by albuminuria:

Category	ACR (mg/mmol)	Approximate ACR (mg/g)	Description
A1	Below 3	Below 30	Normal to mildly increased
A2	3 to 30	30 to 300	Moderately increased (microalbuminuria)
A3	Above 30	Above 300	Severely increased (macroalbuminuria)

These categories combine with the eGFR stage (G1–G5) to define the overall CKD risk level. For example, a patient with G3a (eGFR 45–59) and A2 is at moderate risk; the same eGFR with A3 shifts to high risk.

PCR vs ACR — when to use each

PCR (protein:creatinine ratio) measures total urinary protein and is appropriate for:

Monitoring glomerular disease where heavy, non-albumin proteinuria may be present (for example nephrotic syndrome, IgA nephropathy)
Following proteinuria that has already been confirmed to be significant
Pre-eclampsia screening

ACR (albumin:creatinine ratio) measures only albumin and is the preferred test for:

Screening diabetic nephropathy — albuminuria precedes detectable total proteinuria
Early CKD detection in hypertensive patients
Annual CKD staging according to KDIGO guidelines

Unit conversion: mg/g vs mg/mmol

Some laboratories, particularly in the United States, report ratios in mg per gram of creatinine (mg/g) rather than mg per millimole (mg/mmol). The conversion:

mg/mmol × 8.84 ≈ mg/g
mg/g ÷ 8.84 ≈ mg/mmol

This matters because KDIGO thresholds are defined in mg/mmol internationally. A US lab result of 30 mg/g ACR corresponds to approximately 3.4 mg/mmol — just above the A1/A2 threshold. Enter the appropriate value in the correct unit to avoid misclassification.

Practical guidance for accurate sampling

Prefer an early-morning specimen. The first morning void avoids the effect of orthostatic (postural) proteinuria, where protein leak increases with standing and activity. Postural proteinuria is common and benign in young people, but it can falsely elevate a mid-day spot result.
Both values must be from the same specimen. Mixing protein from one sample with creatinine from another invalidates the ratio.
Units must be consistent. Protein and albumin in mg/L, creatinine in mmol/L for this calculator. If your lab reports creatinine in mg/dL, convert: divide by 11.312 to get mmol/L.
Confirm abnormal results. A single elevated result can reflect recent exercise, fever, or a urinary tract infection. Confirm with a second sample before acting on the result clinically.

When the spot ratio is less reliable

The 10 mmol/day creatinine assumption breaks down in several situations, making the daily estimate less accurate:

Very low muscle mass (frail elderly, malnutrition, limb amputation): creatinine excretion is below 10 mmol/day, overestimating protein-to-creatinine ratio
Very high muscle mass (athletes, bodybuilders): creatinine excretion is above 10 mmol/day, underestimating the ratio
Acute kidney injury with rapidly falling GFR: creatinine excretion is unstable
Very high protein diets: can transiently alter creatinine excretion

In these cases, a timed collection (24-hour or 8-hour) remains the reference standard when the exact figure is clinically important.

This calculator is an educational and clinical aid. It does not constitute medical advice. Clinical decisions should be confirmed with a qualified clinician using the appropriate investigation in context.