Fasting Is Not Required for GFR and Creatinine Testing
No, patients do not need to fast before GFR and creatinine testing, regardless of whether they have kidney disease or diabetes. Patients should maintain their regular diet and scheduled medications, and should drink water to thirst in a pattern consistent with their usual fluid consumption 1, 2.
Standard Testing Protocol
Patient Preparation
- Continue regular diet and medications before the test to determine kidney function in a steady-state setting 1.
- Maintain normal hydration by drinking water to thirst, avoiding both dehydration and overhydration 1, 2.
- No food fasting is required for routine serum creatinine or estimated GFR testing 2.
Specimen Collection
- Spot urine samples (not timed collections) are recommended for albumin-to-creatinine ratio measurement 1.
- First-morning urine specimens are preferred for children and adolescents to avoid confounding effects of orthostatic proteinuria, but random samples are acceptable for adults 1.
- Serum creatinine can be drawn at any time without fasting requirements 1, 2.
Special Circumstances Requiring Consideration
When to Reschedule Testing
- Acute illness (infection, nausea, vomiting, diarrhea) should prompt rescheduling of the test 1.
- Recent iodinated contrast administration requires waiting 7 days before testing to ensure complete washout 1.
Factors That May Transiently Affect Results
- Avoid volume depletion or overhydration immediately before testing, as these can transiently affect GFR 1.
- High protein intake (>70 g/day) may be restricted in specialized GFR measurement protocols using iohexol, but this does not apply to routine creatinine/GFR testing 1.
- Exercise within 24 hours, fever, congestive heart failure, and marked hyperglycemia can affect albumin excretion 1.
Clinical Context and Rationale
Why Fasting Is Not Needed
The measurement of serum creatinine and calculation of estimated GFR are designed to reflect steady-state kidney function 1. Prolonged fasting can lead to volume contraction, which may actually interfere with accurate assessment 1, 2. Water intake does not interfere with these tests and should be encouraged 2.
Confirmation Testing
For routine clinical assessment, estimated GFR based on serum creatinine is sufficient for most clinical decision-making 1, 3. Measured GFR using exogenous markers (like iohexol) is reserved for situations where estimates may be inaccurate, such as extremes of body size, severe malnutrition, rapidly changing kidney function, or when calculating doses of potentially toxic drugs 1, 3.
Common Pitfalls to Avoid
- Do not restrict water intake before testing, as this provides no benefit and may cause unnecessary patient discomfort and physiological stress 2.
- Do not use serum creatinine alone to assess kidney function; always calculate estimated GFR using prediction equations that account for age, sex, race, and body size 1.
- Do not rely on single measurements for diagnosis of chronic kidney disease; abnormal results should be confirmed with repeat testing within 3-6 months 1.