Use of Urine Albumin and Creatinine for Diagnosis of CKD
The urine albumin-to-creatinine ratio (UACR) measured in a random spot urine sample is the first-line test for diagnosing chronic kidney disease in adults. 1
Preferred Testing Method
Obtain a spot urine sample for UACR measurement rather than a 24-hour collection, as the spot test is more convenient, eliminates collection errors, and provides equivalent accuracy for CKD diagnosis. 1
A first-morning void specimen is preferred to minimize variability and exclude orthostatic proteinuria, though a random daytime specimen is acceptable for initial screening. 1, 2
UACR is superior to dipstick urinalysis because dipstick testing has poor sensitivity (43.6%) for detecting clinically significant albuminuria (≥30 mg/g) and produces high false-discovery rates. 3
UACR is preferred over total protein-to-creatinine ratio (UPCR) for initial CKD screening because albumin is the most important protein lost in early kidney disease and UACR more accurately predicts cardiovascular and kidney risks. 1
Diagnostic Thresholds
Normal UACR is <30 mg/g; values ≥30 mg/g indicate abnormal albumin excretion and warrant confirmation. 1
Moderately increased albuminuria (A2 category) is defined as UACR 30–299 mg/g. 1
Severely increased albuminuria (A3 category) is defined as UACR ≥300 mg/g. 1
The term "microalbuminuria" should no longer be used by laboratories; instead, report the numerical UACR value and albuminuria category. 1
Confirmation Requirements
Persistent albuminuria must be confirmed by obtaining two positive results out of three separate samples collected over 3 months before diagnosing CKD, because day-to-day biological variability is substantial. 1, 4
Avoid testing during transient conditions that falsely elevate UACR: vigorous exercise within 24 hours, urinary tract infection, fever, menstruation, marked hyperglycemia, severe hypertension, or heart failure. 1, 5
Combined Assessment with eGFR
Both eGFR and UACR must be quantified to diagnose and stage CKD, as the combination provides superior risk stratification for cardiovascular disease, CKD progression, and mortality compared to either measure alone. 1
Use the CKD-EPI equation to calculate eGFR from serum creatinine, as it is more accurate than the MDRD equation, especially at eGFR ≥60 mL/min/1.73 m². 1
CKD is diagnosed by persistent elevation of UACR, low eGFR (<60 mL/min/1.73 m²), or other manifestations of kidney damage. 1
Screening Recommendations
Screen annually with UACR and eGFR in adults with CKD risk factors: diabetes, hypertension, cardiovascular disease, family history of kidney disease, or age >60 years. 1, 2
In patients with diabetes, screen at diagnosis of type 2 diabetes and after 5 years' duration in type 1 diabetes, then annually thereafter. 1
Common Pitfalls to Avoid
Do not rely on a single UACR measurement to diagnose CKD; day-to-day variability can exceed 170% in the microalbuminuria range, meaning apparent changes may reflect biological variation rather than true disease progression. 4
Do not order 24-hour urine collections routinely; they are cumbersome, prone to incomplete collection (affecting ~30% of samples), and offer no advantage over spot UACR for risk stratification. 1, 5
Do not use urine albumin concentration alone without simultaneous creatinine measurement, as this is susceptible to false results due to hydration variations. 1
Do not dismiss UACR 30–299 mg/g as clinically insignificant; even moderately increased albuminuria independently predicts cardiovascular events and CKD progression at any level of eGFR. 1