Assessing Kidney Function Status
To determine if your kidneys are functioning normally, you need specific laboratory tests—serum creatinine with calculated estimated glomerular filtration rate (eGFR) and urine albumin-to-creatinine ratio (ACR)—as kidney function cannot be assessed by symptoms alone or by a single measurement. 1
Essential Tests Required
You must obtain the following measurements to properly evaluate kidney function:
- Serum creatinine with eGFR calculation using a validated equation (preferably the race-free CKD-EPI equation) 1
- Urine albumin-to-creatinine ratio (ACR) from a spot urine sample 1
- Urinalysis with microscopy to detect proteinuria, hematuria, cellular casts, or other abnormalities 2
A single serum creatinine value alone is insufficient and should never be used to assess kidney function because it is heavily influenced by muscle mass, age, sex, nutritional status, and certain medications. 3, 2
Interpreting Your Results
Normal Kidney Function Defined As:
- eGFR ≥90 mL/min/1.73 m² (Category G1) with no evidence of kidney damage 1
- Urine ACR <30 mg/g or <3 mg/mmol (Category A1) 1
- No structural abnormalities on imaging if performed 1
Abnormal Kidney Function Indicators:
Chronic Kidney Disease (CKD) is diagnosed when either of these criteria persists for >3 months: 1, 4
- eGFR <60 mL/min/1.73 m² (Categories G3a through G5), OR
- Albuminuria ≥30 mg/g or ≥3 mg/mmol (Categories A2 or A3), OR
- Other markers of kidney damage (abnormal urinalysis, structural abnormalities)
The GFR categories are: 1
- G1: ≥90 mL/min/1.73 m² (normal to increased)
- G2: 60-89 mL/min/1.73 m² (mildly reduced)
- G3a: 45-59 mL/min/1.73 m² (moderately reduced)
- G3b: 30-44 mL/min/1.73 m² (moderately reduced)
- G4: 15-29 mL/min/1.73 m² (severely reduced)
- G5: <15 mL/min/1.73 m² or on dialysis (kidney failure)
The albuminuria categories are: 1
- A1: <30 mg/g (normal to mildly increased)
- A2: 30-300 mg/g (moderately increased)
- A3: >300 mg/g (severely increased)
Critical Pitfalls to Avoid
Serum Creatinine Limitations:
Serum creatinine does not increase linearly with falling GFR—your GFR can fall to approximately half of normal before creatinine rises above the normal range. 3 This means you could have significant kidney dysfunction with a "normal" creatinine level.
Creatinine is falsely elevated by: 3, 2
- High muscle mass or creatine supplementation
- Intense exercise
- Certain medications (trimethoprim, cimetidine)
Creatinine is falsely lowered by: 3
- Low muscle mass
- Advanced age
- Malnutrition
- Amputation
Single Measurement Inadequacy:
One abnormal test does not establish CKD diagnosis—abnormalities must persist for at least 3 months. 1, 2, 4 Repeat testing is mandatory before diagnosing chronic kidney disease.
eGFR calculations are only valid in steady-state conditions and cannot assess acute changes in kidney function. 2
When Additional Testing Is Needed
Consider cystatin C-based eGFR if creatinine-based estimates are unreliable due to: 2
- Extremes of muscle mass
- Recent creatine supplementation
- Amputation or paralysis
Consider renal ultrasound with Doppler if CKD is confirmed to: 1
- Assess kidney structure
- Determine causes of CKD
- Exclude renovascular disease
Monitoring Frequency
If initial tests are normal: No specific monitoring interval is required unless you develop risk factors (diabetes, hypertension, cardiovascular disease). 1, 4
If moderate-to-severe CKD is diagnosed (eGFR <60 or ACR ≥30): Repeat measurements at least annually, though every 2-4 months is typical during stable phases. 1, 3
If you are on nephrotoxic medications or ACE inhibitors/ARBs: Test 1-2 weeks after initiation, then 1-2 weeks after each dose titration. 3
Bottom Line
You cannot know if your kidneys are "ok" without obtaining serum creatinine with calculated eGFR and urine albumin-to-creatinine ratio. 1 These tests must show eGFR ≥60 mL/min/1.73 m² and ACR <30 mg/g, with no other markers of kidney damage, to confirm normal kidney function. If either value is abnormal, repeat testing after 3 months is required to determine if chronic kidney disease is present. 1, 4