Testes de função renal: BUN, creatinina e TFGe explicados
Resposta rápida: A função renal é avaliada principalmente pela creatinina (Creatinine), nitrogênio ureico sanguíneo (BUN, Blood Urea Nitrogen) e taxa de filtração glomerular estimada (TFGe). Valores normais de referência: creatinina homens 0,7-1,3mg/dL, mulheres 0,6-1,1mg/dL; BUN 7-20mg/dL; TFGe >90mL/min/1,73m² é normal. Estágios da doença renal crônica (DRC): estágio 1 TFGe>90 (com marcadores de lesão renal), estágio 2: 60-89, estágio 3a: 45-59, estágio 3b: 30-44, estágio 4: 15-29, estágio 5: <15 (necessita diálise). Uma relação BUN/creatinina >20:1 sugere causa pré-renal (desidratação, insuficiência cardíaca); se ambos estiverem elevados com relação normal, sugere doença renal parenquimatosa. Fatores não renais que afetam a creatinina: massa muscular (valores mais altos em esportistas), dieta hiperproteica, suplementos de creatina. A relação albumina/creatinina na urina (RAC) é um indicador sensível de lesão renal precoce. Recomenda-se controle anual em pessoas de alto risco (diabetes, hipertensão).
Disclaimer: Este conteúdo é apenas informativo e não constitui aconselhamento médico. Aviso legal completo.
What Do Kidney Function Tests Measure?
The kidneys filter approximately 180 liters of blood daily, removing waste products and maintaining fluid/electrolyte balance. Kidney function tests assess how well this filtration system is working:
- Creatinine: A waste product from normal muscle metabolism, filtered entirely by the kidneys. Normal: 0.7-1.3 mg/dL for men, 0.6-1.1 mg/dL for women. Higher levels indicate reduced kidney filtration. However, creatinine is also influenced by muscle mass — a muscular person may have higher creatinine without kidney disease.
- BUN (Blood Urea Nitrogen): A waste product from protein metabolism. Normal: 7-20 mg/dL. BUN is less specific than creatinine because it is also affected by protein intake, dehydration, liver function, and GI bleeding.
- eGFR (Estimated Glomerular Filtration Rate): Calculated from creatinine, age, sex, and (in some equations) race. This is the most clinically useful measure of kidney function.
eGFR staging of kidney function:
- Stage 1: eGFR 90+ (normal function, but other signs of kidney damage present)
- Stage 2: eGFR 60-89 (mildly decreased)
- Stage 3a: eGFR 45-59 (mild-to-moderate decrease)
- Stage 3b: eGFR 30-44 (moderate-to-severe decrease)
- Stage 4: eGFR 15-29 (severely decreased)
- Stage 5: eGFR below 15 (kidney failure)
Upload your kidney function results to WAYJET's Medical Report Analyzer for a comprehensive assessment.
What Causes Abnormal Kidney Function Tests?
Elevated creatinine or reduced eGFR can result from various causes:
Acute causes (reversible):
- Dehydration (most common cause of mildly elevated creatinine)
- NSAID use (ibuprofen, naproxen reduce renal blood flow)
- Contrast dye from imaging procedures
- Urinary tract obstruction (kidney stones, enlarged prostate)
- Acute infection or illness
Chronic causes:
- Diabetes (the leading cause of chronic kidney disease, accounting for 44% of cases)
- Hypertension (the second leading cause)
- Glomerulonephritis (autoimmune kidney inflammation)
- Polycystic kidney disease (genetic)
- Long-term NSAID or PPI use
An elevated BUN:creatinine ratio (above 20:1) suggests pre-renal causes like dehydration, GI bleeding, or high protein intake. A ratio below 10:1 may indicate liver disease or malnutrition.
Important: a single abnormal result does not diagnose kidney disease. Chronic kidney disease requires demonstrated abnormality lasting 3+ months. Always retest before drawing conclusions.
How Can You Protect Your Kidneys?
Evidence-based kidney protection strategies are especially important for those with diabetes, hypertension, or existing CKD:
- Blood pressure control: The most important modifiable factor. Target below 130/80 mmHg for kidney protection. ACE inhibitors and ARBs have specific kidney-protective effects beyond blood pressure lowering.
- Blood sugar control: HbA1c below 7% significantly reduces diabetic nephropathy progression.
- Hydration: Adequate water intake (2-3 liters daily for most adults) supports kidney function. Chronic dehydration concentrates toxins and increases kidney workload.
- Limit NSAIDs: Regular NSAID use reduces kidney blood flow. Use acetaminophen as a first-line pain reliever when possible.
- Moderate protein intake: For established CKD (stage 3+), protein restriction to 0.6-0.8g/kg daily may slow progression. For healthy kidneys, higher protein intake is not harmful.
- Annual screening: If you have diabetes, hypertension, or family history of kidney disease, annual creatinine/eGFR and urine albumin-to-creatinine ratio (UACR) screening is recommended. UACR above 30 mg/g is an early sign of kidney damage, often appearing before eGFR changes.
Perguntas frequentes
Can dehydration cause elevated creatinine?
Yes, dehydration is the most common cause of mildly elevated creatinine in otherwise healthy people. When dehydrated, less blood flows through the kidneys, reducing filtration and causing creatinine to accumulate temporarily. Always hydrate well before blood work. If creatinine was mildly elevated, retest when well-hydrated before assuming kidney disease.
Does high protein intake damage kidneys?
In people with healthy kidneys, high protein intake (up to 2g/kg) has not been shown to cause kidney damage in any long-term study. However, for people with existing chronic kidney disease (eGFR below 60), high protein intake may accelerate progression. The distinction between healthy kidneys and diseased kidneys is critical.
What does it mean if eGFR decreases with age?
eGFR naturally declines by about 1 mL/min per year after age 30. An eGFR of 70 in a 75-year-old may represent normal aging rather than kidney disease. Context matters — your doctor will consider age, trends over time, proteinuria, and other markers before diagnosing CKD. A single low eGFR reading in an older adult does not necessarily require treatment.
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