How to interpret kft
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KFT kidney function test interpretation normal values creatinine BUN GFR
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I now have sufficient textbook material to give a thorough, well-structured answer. Here is a comprehensive guide to interpreting a KFT (Kidney Function Test):
Interpreting a KFT (Kidney Function Test)
A KFT is a panel of blood and urine tests that together assess how well the kidneys are filtering waste, regulating electrolytes, and maintaining fluid balance. The functional unit evaluated is the nephron - since most kidney disease reduces the number of functioning nephrons rather than impairing each nephron individually, tests that reflect overall filtration capacity are the most useful.
1. Serum Creatinine
| Parameter | Normal Range |
|---|---|
| Men | 0.7 - 1.2 mg/dL |
| Women | 0.5 - 1.0 mg/dL |
What it measures: Creatinine is a breakdown product of creatine phosphate in muscle. It is freely filtered by the glomerulus and not reabsorbed, making it a reliable marker of GFR.
How to interpret:
- Elevated creatinine = reduced GFR, suggesting impaired kidney filtration
- A significant rise in serum creatinine is only seen after ~50% decline in GFR - it is an insensitive early marker
- Creatinine levels are influenced by muscle mass, age, sex, race, and dietary meat intake - a muscular young man may have a high-normal creatinine while an elderly woman with poor muscle mass may have a low creatinine despite significantly reduced kidney function
- Drugs like cimetidine, trimethoprim, and salicylates can block tubular secretion of creatinine, falsely elevating levels
- The creatinine-GFR relationship is non-linear (parabolic): at higher GFRs, a large drop in filtration causes only a small rise in creatinine; at lower GFRs, small drops in filtration cause large creatinine spikes
Source: Textbook of Family Medicine, 9e; Tietz Textbook of Laboratory Medicine, 7th Ed
2. Blood Urea Nitrogen (BUN) / Serum Urea
| Parameter | Normal Range |
|---|---|
| BUN | 7 - 20 mg/dL |
| Serum Urea | 15 - 45 mg/dL |
What it measures: Urea is the end product of protein metabolism, synthesized in the liver and excreted by the kidneys.
How to interpret:
- Elevated BUN suggests reduced kidney filtration, but BUN is less specific than creatinine for renal function because it is also affected by:
- High protein diet (raises BUN)
- Dehydration or hypovolemia (raises BUN via increased tubular reabsorption - called "prerenal azotemia")
- GI bleeding (protein from blood is digested and absorbed, raising BUN)
- Liver failure (decreases BUN production, may mask elevation)
- Catabolic states (sepsis, burns) - raise BUN
BUN:Creatinine Ratio - a very useful interpretive tool:
| Ratio | Interpretation |
|---|---|
| 10:1 - 20:1 | Normal |
| > 20:1 | Prerenal cause (dehydration, poor cardiac output, GI bleed) |
| < 10:1 | Intrinsic renal disease, liver failure, or low protein intake |
Source: Harrison's Principles of Internal Medicine, 22nd Ed; Tietz Textbook of Laboratory Medicine
3. Estimated GFR (eGFR)
| eGFR (mL/min/1.73 m²) | Interpretation |
|---|---|
| ≥ 90 | Normal kidney function |
| 60 - 89 | Mildly reduced (CKD Stage 1-2 if other markers present) |
| 30 - 59 | Moderately reduced (CKD Stage 3) |
| 15 - 29 | Severely reduced (CKD Stage 4) |
| < 15 | Kidney failure - may require dialysis |
GFR is the gold standard for assessing kidney function. Commonly used estimating equations:
- Cockcroft-Gault:
eGFR (mL/min) = (140 - age) × weight (kg) / (Cr × 72)[× 0.85 for females] - MDRD equation:
GFR = 186 × (serum Cr)^-1.154 × (age)^-0.203[× 0.742 for females; × 1.212 for Black patients] - most accurate when GFR < 60 - CKD-EPI: Most accurate across all GFR ranges, recommended by current guidelines
Note: GFR declines ~10% per decade after age 30, and is normally ~10 mL/min higher in men than women. - Barash Clinical Anesthesia, 9e
4. Urine Protein / Albumin (Proteinuria)
| Measurement | Normal |
|---|---|
| Total protein | < 150 mg/24 hrs |
| Urine albumin | < 30 mg/24 hrs |
| Albumin:Creatinine Ratio (ACR) | < 3 mg/mmol |
Interpretation:
- Microalbuminuria (30-300 mg/day): Early marker of renal damage - especially important in diabetes and hypertension. Screening begins 5 years after Type 1 DM diagnosis and at diagnosis for Type 2 DM
- Macroproteinuria (> 300 mg/day): Suggests significant glomerular damage (nephrotic syndrome, diabetic nephropathy, SLE nephritis)
- Proteinuria + reduced eGFR together define and stage Chronic Kidney Disease (CKD)
- False positives: orthostatic proteinuria, exercise, UTI, menstrual contamination, heart failure
Source: Tietz Textbook of Laboratory Medicine, 7th Ed; Harper's Illustrated Biochemistry, 32nd Ed
5. Serum Uric Acid
| Parameter | Normal Range |
|---|---|
| Men | 3.5 - 7.2 mg/dL |
| Women | 2.6 - 6.0 mg/dL |
- Elevated uric acid (hyperuricemia) can indicate impaired renal excretion, gout, or high cell turnover (chemotherapy, lymphoma)
- Serum uric acid is elevated in 90% of patients with gout, but the definitive gout diagnosis requires demonstration of urate crystals in synovial fluid
- Treatment target for urate-lowering therapy: < 6 mg/dL (or < 5 mg/dL in severe disease)
- In kidney disease, reduced uric acid excretion causes hyperuricemia, which in turn may have direct pathogenic effects on the kidney
Source: Rheumatology (Elsevier, 2022)
6. Electrolytes (often included in KFT)
| Electrolyte | Normal Range | Significance in Renal Disease |
|---|---|---|
| Sodium (Na+) | 135 - 145 mEq/L | Hyponatremia in SIADH, hypernatremia in dehydration |
| Potassium (K+) | 3.5 - 5.0 mEq/L | Hyperkalemia is a key danger in CKD/AKI - can cause fatal arrhythmias |
| Bicarbonate (HCO3-) | 22 - 29 mEq/L | Low in metabolic acidosis (common in CKD) |
| Phosphate | 2.5 - 4.5 mg/dL | Elevated in CKD (kidney can't excrete phosphate) |
| Calcium | 8.5 - 10.5 mg/dL | Often low in CKD (reduced vitamin D activation) |
7. Urinalysis (Dipstick + Microscopy)
| Finding | Significance |
|---|---|
| Blood/RBCs | Glomerulonephritis, UTI, stones, tumors |
| RBC casts | Pathognomonic of glomerulonephritis |
| WBC casts | Pyelonephritis, interstitial nephritis |
| Granular/waxy casts | Advanced CKD |
| Glucose without hyperglycemia | Tubular dysfunction (Fanconi syndrome) |
| Specific gravity < 1.003 | Dilute urine, impaired concentrating ability |
Summary: Quick Clinical Interpretation
| Scenario | Pattern |
|---|---|
| Acute Kidney Injury (AKI) | Sudden rise in creatinine ± oliguria, BUN:Cr ratio varies by cause |
| Prerenal AKI (dehydration) | BUN:Cr > 20:1, high urine specific gravity, concentrated urine |
| Intrinsic renal AKI | BUN:Cr ~ 10-15:1, granular casts, hematuria |
| CKD | Slow ↑ creatinine + ↓ eGFR + persistent proteinuria + anemia + hyperkalemia + ↑ phosphate |
| Nephrotic syndrome | Heavy proteinuria (> 3.5 g/day), hypoalbuminemia, edema, hyperlipidemia |
| Nephritic syndrome | Hematuria, RBC casts, hypertension, mild proteinuria, ↑ creatinine |
A CKD diagnosis requires both eGFR < 60 mL/min/1.73 m² and/or markers of kidney damage (albuminuria, abnormal urinalysis) persisting for > 3 months. An eGFR below 15 is associated with uremic symptoms and is the threshold at which dialysis is typically considered. - Barash Clinical Anesthesia, 9e; Tietz Textbook of Laboratory Medicine, 7th Ed
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