uACR
Urine Albumin-to-Creatinine Ratio
What it is
The urine albumin-to-creatinine ratio (uACR) measures the amount of albumin in a spot urine sample, normalised against urine creatinine concentration to correct for variable urine dilution. Albumin is a large plasma protein that is normally excluded from the urine by the glomerular filtration barrier — a specialised structure of the kidney that filters blood while retaining large molecules. When the glomerular barrier is damaged or its electrical charge is disrupted, albumin leaks through into the filtrate and appears in the urine. The urinary creatinine concentration corrects for the fact that a dilute urine sample (from high fluid intake) will have lower albumin simply due to dilution — expressing the result as a ratio produces a measurement that is independent of hydration status. uACR therefore detects glomerular barrier dysfunction at a stage when the overall filtration rate (measured by eGFR and serum creatinine) remains entirely normal — making it the earliest available blood or urine marker of kidney damage.
Why we measure it
The clinical significance of uACR extends beyond kidney disease. Microalbuminuria — a uACR between 3 and 30 mg/mmol — was originally characterised in patients with type 1 diabetes as the earliest predictor of diabetic nephropathy, preceding overt proteinuria and GFR decline by years. This finding was replicated in type 2 diabetes and hypertension, establishing microalbuminuria as a critical early warning of nephropathy in both conditions. But subsequent large cohort studies demonstrated that elevated uACR is also a potent independent predictor of cardiovascular events — myocardial infarction, stroke, and cardiovascular death — in the general population, including people without diabetes or hypertension. The HOPE study (Yusuf et al., NEJM 2001) found that microalbuminuria was an independent predictor of major cardiovascular events in a broad population of high-risk adults. The mechanisms are shared: endothelial dysfunction that produces glomerular albumin leak is the same process that underlies early atherosclerosis. uACR thus serves as a window into systemic endothelial health. In the Blueprint protocol, uACR measured quarterly alongside Cystatin-C gives a complete picture of renal health — Cystatin-C captures glomerular filtration rate; uACR captures glomerular barrier integrity. Both can be abnormal independently, and together they detect different patterns of renal vulnerability.
Why every 3 months
uACR responds to blood pressure control, glycaemic management, dietary sodium restriction, and medications — particularly ACE inhibitors and ARBs, which have specific renoprotective effects at the glomerulus — within weeks to months. This responsiveness makes quarterly monitoring valuable for individuals with identified risk factors (diabetes, hypertension, metabolic syndrome) and for detecting the onset of microalbuminuria in the general population before it progresses. uACR also has meaningful day-to-day biological variability — strenuous exercise, fever, or orthostatic stress can transiently elevate it. The trend across multiple quarterly draws, rather than any single reading, is the reliable signal.
What movement means
The KDIGO (Kidney Disease: Improving Global Outcomes) classification and the ADA define albuminuria categories based on uACR. Values are reported in mg/mmol (SI) or mg/g (conventional). A single elevated reading should be confirmed on a repeat measurement, as transient elevation is common.
Normal
< 3 mg/mmol (< 30 mg/g)
Normal glomerular albumin excretion. No evidence of albuminuria. Annual retesting is appropriate in low-risk individuals; quarterly monitoring in those with diabetes, hypertension, or metabolic syndrome.
KDIGO 2012 Clinical Practice Guideline for Chronic Kidney Disease
Microalbuminuria
3 – 30 mg/mmol (30 – 300 mg/g)
Moderately increased albuminuria. Indicates early glomerular barrier disruption. Substantially elevated risk of CKD progression and cardiovascular events. Warrants blood pressure optimisation, glycaemic control if applicable, and consideration of ACE inhibitor or ARB therapy. Confirm on repeat measurement before initiating intervention.
KDIGO 2012 Clinical Practice Guideline for Chronic Kidney Disease — doi:10.1038/kisup.2012.73
Macroalbuminuria
> 30 mg/mmol (> 300 mg/g)
Severely increased albuminuria. Indicates significant glomerular pathology. Associated with high risk of progressive CKD and cardiovascular events. Warrants nephrology review and aggressive management of all modifiable risk factors.
KDIGO 2012 Clinical Practice Guideline for Chronic Kidney Disease — doi:10.1038/kisup.2012.73
In the protocol
References
- 1.
Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. “KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.” Kidney International Supplements. 2013.
- 2.
Yusuf S, Sleight P, Pogue J, Bosch J, Davies R, Dagenais G. “Effects of an Angiotensin-Converting-Enzyme Inhibitor, Ramipril, on Cardiovascular Events in High-Risk Patients (HOPE Study).” New England Journal of Medicine. 2000.
- 3.
Gerstein HC, Mann JFE, Yi Q, et al.. “Albuminuria and Risk of Cardiovascular Events, Death, and Heart Failure in Diabetic and Nondiabetic Individuals.” JAMA. 2001.
- 4.
Levey AS, de Jong PE, Coresh J, et al.. “The Definition, Classification, and Prognosis of Chronic Kidney Disease: A KDIGO Controversies Conference Report.” Kidney International. 2011.