Guideline Recommendations for Urine Albumin‑to‑Creatinine Ratio (ACR) Assessment
Normal and Abnormal ACR Thresholds
- The National Kidney Foundation (NKF) and NIDDK define a normal spot‑urine ACR as ≤ 30 mg albumin per gram creatinine; values above this indicate albuminuria【1】.
- Microalbuminuria (moderately increased albuminuria) is defined as an ACR of 31–299 mg/g【1】.
- Macroalbuminuria (severely increased albuminuria) is defined as an ACR of ≥ 300 mg/g【1】.
Equivalent Thresholds for Different Collection Methods
| Collection type | Normal range | Microalbuminuria range | Clinical (macro) range |
|---|---|---|---|
| 24‑hour urine (mg/24 h) | < 30 | 30–299 | ≥ 300【5】 |
| Timed urine (μg/min) | < 20 | 20–199 | ≥ 200【5】 |
These equivalencies allow clinicians to interpret results from 24‑hour or timed collections in the same clinical context as spot‑urine ACR【5】.
Specimen Collection and Confirmation Protocols
- First‑morning void specimens are preferred because they reduce diurnal variation in albumin excretion【1】.
- If a first‑morning sample cannot be obtained, all collections should be performed at the same time of day, with the patient well‑hydrated and having avoided food, exercise, or heavy exertion for at least 2 hours【9】.
- To confirm persistent albuminuria, 2 of 3 specimens collected over a 3–6 month period must show an ACR > 30 mg/g【1】.
- Transient elevations can occur with vigorous exercise (within 24 h), urinary tract infection, fever, congestive heart failure, marked hyperglycemia, marked hypertension, pyuria, or hematuria; patients should refrain from vigorous exercise for 24 h before sampling【1】.
Preferred Measurement Method and Laboratory Considerations
- Spot‑urine ACR is the preferred clinical method over 24‑hour or timed collections because it corrects for urinary concentration and is more convenient【1】【8】.
- The ratio is calculated as milligrams of albumin per gram of creatinine【1】.
- Urine creatinine is reported in grams per deciliter and serves as a correction factor, eliminating the need for timed collections【1】【8】.
- Immunoassays for albumin have sufficient sensitivity, and urine creatinine assays are fairly well standardized, supporting reliable ACR measurement in routine practice【1】.
- Timed collections should be reserved for research settings rather than routine clinical care【9】.