Kft interpretation
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Now I have comprehensive information to provide a thorough KFT interpretation guide.
Kidney Function Tests (KFT) - Complete Interpretation Guide
KFT (also called Renal Function Tests / RFT) is a panel of blood and urine investigations used to assess how well the kidneys are filtering waste, regulating electrolytes, and maintaining fluid balance. Here is how each component is interpreted:
1. Serum Creatinine
| Reference Range | Interpretation |
|---|---|
| Males: 0.7-1.3 mg/dL | Normal |
| Females: 0.5-1.1 mg/dL | Normal |
| Elevated | Reduced GFR / renal impairment |
Key points:
- Creatinine is a waste product of muscle metabolism. Serum levels depend on muscle mass, age, gender, race, and dietary meat intake.
- Normal serum creatinine ranges from 0.5 mg/dL (thin/elderly women) to 1.5 mg/dL (muscular men) - Goldman-Cecil Medicine
- Creatinine rises in a parabolic fashion as GFR falls - large drops in GFR cause small creatinine changes at higher GFR, but small drops cause large creatinine changes at low GFR
- Drugs that falsely elevate creatinine (by blocking tubular secretion): cimetidine, trimethoprim, fenofibrate, salicylates
- Serum creatinine remains normal with unilateral renal pathology if the contralateral kidney functions normally - Bailey & Love's Surgery
2. Blood Urea Nitrogen (BUN)
| Reference Range | Value |
|---|---|
| Normal | 7-20 mg/dL (2.5-7.1 mmol/L) |
- BUN is a product of protein catabolism processed by the liver and excreted by the kidneys
- BUN:Creatinine Ratio (normal = 10:1 to 20:1)
- >20:1 (prerenal): dehydration/hypovolemia, upper GI bleed, high protein diet, corticosteroid use
- <10:1: malnutrition, liver disease (reduced urea production), low protein intake
- 10-20:1: intrinsic renal disease (ratio usually stays "normal" proportionally)
"In volume-contracted states, this ratio may increase because of an associated differential increase in urea reabsorption in the collecting duct." - Comprehensive Clinical Nephrology, 7th Ed.
3. eGFR (Estimated Glomerular Filtration Rate)
eGFR is the most clinically informative marker of overall kidney function. The CKD-EPI equation is now the preferred formula (replaced MDRD), using serum creatinine, age, and sex.
KDIGO CKD Staging by eGFR
| Stage | eGFR (mL/min/1.73 m²) | Description |
|---|---|---|
| G1 | ≥90 | Normal or high |
| G2 | 60-89 | Mildly decreased |
| G3a | 45-59 | Mildly to moderately decreased |
| G3b | 30-44 | Moderately to severely decreased |
| G4 | 15-29 | Severely decreased |
| G5 | <15 | Kidney failure |
CKD is defined as eGFR <60 mL/min/1.73m² persisting for >3 months, OR evidence of kidney damage (proteinuria, hematuria, structural abnormality). - Comprehensive Clinical Nephrology, 7th Ed.
Schwartz equation is used for children:
eGFR = 0.413 × Height (cm) / Serum Creatinine (mg/dL)
Cystatin C is an emerging, more accurate GFR marker - not influenced by muscle mass, freely filtered, not tubularly secreted. Now incorporated into updated CKD-EPI equations.
4. Serum Urea (BUN)
Often reported as urea (mmol/L) in many countries, or BUN (mg/dL) in others.
- Conversion: BUN (mg/dL) = Urea (mmol/L) × 2.8
- Elevated in: dehydration, high protein intake, GI bleed, renal failure, catabolic states
- Low in: liver disease, malnutrition, overhydration
5. Serum Electrolytes
| Electrolyte | Normal Range | Clinical Notes |
|---|---|---|
| Sodium (Na⁺) | 136-145 mEq/L | Hyponatremia in nephrotic syndrome, SIADH; hypernatremia in dehydration |
| Potassium (K⁺) | 3.5-5.0 mEq/L | Hyperkalemia is a key complication of renal failure - can be fatal |
| Bicarbonate (HCO₃⁻) | 22-29 mEq/L | Low in metabolic acidosis of CKD |
| Chloride (Cl⁻) | 96-106 mEq/L | Interpreted with Na and bicarb for acid-base status |
6. Serum Uric Acid
| Reference Range | Value |
|---|---|
| Males | 3.4-7.0 mg/dL |
| Females | 2.4-6.0 mg/dL |
- Elevated in gout, renal impairment, diuretic use, high purine diet
- Uric acid is filtered and partially reabsorbed/secreted by the tubules
7. Serum Calcium & Phosphorus
These are routinely checked in renal panels because the kidneys regulate their balance:
- Calcium (normal: 8.5-10.5 mg/dL): Low serum albumin artificially lowers measured calcium - correct by adding 0.8 mg/dL for every 1 g/dL albumin is below 4 g/dL
- Phosphorus (normal: 2.5-4.5 mg/dL): Rises in CKD as phosphate excretion fails
- In CKD, the pattern is typically: low calcium + high phosphorus + elevated PTH (secondary hyperparathyroidism)
8. Urine Analysis Components
| Test | Normal | Significance |
|---|---|---|
| Urine specific gravity | 1.005-1.030 | Low in renal tubular disease, DI; high in dehydration |
| Proteinuria | <150 mg/day (trace or absent) | Key marker of glomerular damage; >3.5 g/day = nephrotic range |
| Hematuria | Absent | Glomerulonephritis, stones, malignancy |
| Urine sodium | Variable | Low (<20 mEq/L) in prerenal; high (>40 mEq/L) in ATN |
| FENa | - | <1% = prerenal; >1% = intrinsic renal (ATN) |
Fractional Excretion of Sodium (FENa):
FENa = (U_Na × P_Cr) / (U_Cr × P_Na) × 100
9. Creatinine Clearance (CrCl)
Calculated from a 24-hour urine collection:
CrCl = (U_Cr × Urine flow rate) / P_Cr
- Estimates GFR but overestimates it slightly due to tubular secretion of creatinine
- Cockcroft-Gault equation estimates CrCl without urine collection - used for drug dosing adjustments
Summary: Quick Clinical Clues
| Pattern | Likely Cause |
|---|---|
| High creatinine + low eGFR + high K⁺ + metabolic acidosis | CKD/AKI |
| High BUN:Cr ratio (>20) + low urine Na (<20) | Prerenal (dehydration/hypovolemia) |
| Normal BUN:Cr + elevated creatinine + urine Na >40 | Intrinsic renal disease (ATN) |
| Nephrotic: heavy proteinuria + low albumin + edema | Glomerular disease |
| Low Ca²⁺ + high PO₄ + high PTH + anemia | CKD with secondary hyperparathyroidism |
Sources:
- Campbell-Walsh-Wein Urology
- Comprehensive Clinical Nephrology, 7th Edition
- Goldman-Cecil Medicine
- Textbook of Family Medicine, 9th Edition
- Bailey & Love's Short Practice of Surgery, 28th Edition
What is acute kidney injury
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"acute kidney injury"[MeSH Terms] AND "diagnosis"[MeSH Terms]
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I now have comprehensive information. Here is a thorough guide on Acute Kidney Injury:
Acute Kidney Injury (AKI)
Definition
AKI is defined by the KDIGO (Kidney Disease: Improving Global Outcomes) criteria as any of the following:
- Rise in serum creatinine ≥0.3 mg/dL within 48 hours, OR
- Rise in serum creatinine to ≥1.5× baseline within 7 days, OR
- Urine output <0.5 mL/kg/h for ≥6 hours
- National Kidney Foundation Primer on Kidney Diseases, 8th Ed.
KDIGO Staging
| Stage | Serum Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| 1 | ↑ ≥0.3 mg/dL OR ↑ ×1.5-1.9 from baseline | <0.5 mL/kg/h for 6-12 h |
| 2 | ↑ ×2.0-2.9 from baseline | <0.5 mL/kg/h for ≥12 h |
| 3 | ↑ ×3.0 OR ≥4.0 mg/dL OR initiation of RRT* | <0.3 mL/kg/h for ≥24 h OR anuria ≥12 h |
*In patients <18 years: eGFR <35 mL/min/1.73m² also qualifies for Stage 3.
Patients who receive kidney replacement therapy (KRT) are automatically classified as Stage 3 regardless of their creatinine at the time KRT is started.
Classification by Cause
AKI is classically divided into three anatomical categories:
1. Prerenal AKI (~21% of hospital AKI)
Caused by reduced renal perfusion - GFR falls but the kidney itself is structurally intact initially.
Causes:
- True hypovolemia: hemorrhage, GI losses (vomiting/diarrhea), burns, diuretic overuse
- Reduced cardiac output: congestive heart failure, cardiogenic shock
- Systemic vasodilation: sepsis, liver failure (hepatorenal syndrome), anaphylaxis
- Renal vasoconstriction: NSAIDs, ACE inhibitors/ARBs in the setting of hypotension, contrast nephropathy
Key diagnostic features:
- BUN:Creatinine ratio >20:1
- Urine sodium <20 mEq/L
- FENa <1%
- Urine specific gravity high (concentrated urine)
- Reversible within 24-72 hours of fluid resuscitation/correction
"The hallmark of prerenal disease is its reversibility after treatment of the underlying cause, with absence of structural damage to the kidney if treated promptly." - Campbell-Walsh-Wein Urology
2. Intrinsic (Renal) AKI (~58% of hospital AKI)
Structural damage to the kidney itself.
| Compartment | Disease | Features |
|---|---|---|
| Tubules | Acute Tubular Necrosis (ATN) - most common (45%) | Urine Na >40, FENa >1%, muddy brown casts |
| Glomeruli | Glomerulonephritis, TTP/HUS | RBC casts, heavy proteinuria |
| Interstitium | Acute Interstitial Nephritis (AIN) | WBC casts, eosinophilia, drug/infection cause |
| Vessels | Renal artery/vein thrombosis, vasculitis |
Common causes of ATN:
- Ischemic: Prolonged hypotension, septic shock
- Nephrotoxic drugs: Aminoglycosides, amphotericin B, cisplatin, vancomycin, NSAIDs, contrast agents, cyclosporine, tenofovir
- Endogenous toxins: Myoglobin (rhabdomyolysis), hemoglobin (hemolysis), uric acid (tumor lysis syndrome)
Newer nephrotoxic agents to be aware of:
- Checkpoint inhibitors (nivolumab, pembrolizumab) - cause AIN
- Tyrosine kinase inhibitors (sorafenib, sunitinib) - cause TMA and proteinuria
3. Postrenal AKI (~10% of hospital AKI)
Caused by obstruction at any level of the urinary tract. Requires bilateral obstruction (or unilateral in a solitary kidney) to cause AKI.
Causes:
- Benign prostatic hyperplasia (most common in older men)
- Bilateral ureteral stones
- Pelvic/retroperitoneal malignancy
- Neurogenic bladder
- Strictures
Clinical Features
AKI is often asymptomatic in early stages. Symptoms arise from the metabolic consequences:
- Oliguria (urine output <400 mL/day) or anuria
- Fluid overload: edema, pulmonary edema, hypertension
- Uremic symptoms: nausea, vomiting, hiccups, metallic taste, encephalopathy, asterixis
- Fatigue, decreased appetite
Complications
| System | Complication |
|---|---|
| Metabolic | Hyperkalemia, metabolic acidosis (↑anion gap), hyponatremia, hypocalcemia, hyperphosphatemia, hyperuricemia, hypermagnesemia |
| Cardiovascular | Pulmonary edema, arrhythmias (from hyperkalemia), pericarditis, pericardial effusion, hypertension, MI |
| Gastrointestinal | Nausea, vomiting, GI hemorrhage, malnutrition |
| Neurologic | Encephalopathy, asterixis, seizures, mental status changes |
| Hematologic | Anemia, uremic bleeding (platelet dysfunction) |
| Infectious | Pneumonia, septicemia, UTI |
- Brenner & Rector's The Kidney, 2-Volume Set
Diagnosis: Key Lab Differentiation
| Test | Prerenal | Intrinsic (ATN) | Postrenal |
|---|---|---|---|
| BUN:Cr ratio | >20:1 | ~10-15:1 | Variable |
| Urine Na (mEq/L) | <20 | >40 | >40 |
| FENa | <1% | >1-2% | >1% |
| Urine osmolality | >500 mOsm/kg | <350 mOsm/kg | Variable |
| Urine specific gravity | >1.020 | ~1.010 (isosthenuric) | Variable |
| Urinary casts | Hyaline casts | Muddy brown granular casts | None or RBC/WBC |
Management
General Supportive Measures
- Treat the underlying cause - fluid resuscitation for hypovolemia, antibiotics for sepsis, relieve obstruction for postrenal
- Stop nephrotoxins - hold ACE inhibitors, ARBs, NSAIDs, contrast agents, aminoglycosides
- Optimize fluid balance - avoid both over- and under-hydration
- Nutrition: KDIGO recommends 20-30 kcal/kg/day; protein 0.8-1.0 g/kg/day (non-dialysis), 1.0-1.5 g/kg/day (dialysis), up to 1.7 g/kg/day (hypercatabolic on CRRT)
- Metabolic acidosis: Treat only if pH <7.20 and HCO₃⁻ <15 mmol/L with IV/oral sodium bicarbonate
- Hyperphosphatemia: Phosphate binders (calcium carbonate, sevelamer, lanthanum)
- DVT prophylaxis - avoid LMWH and factor Xa inhibitors in severe AKI; prefer unfractionated heparin
Renal Replacement Therapy (RRT)
Indications (the "AEIOU" mnemonic):
- A - Acidosis (severe, refractory to bicarbonate)
- E - Electrolytes (hyperkalemia refractory to medical management)
- I - Intoxications (certain toxic ingestions)
- O - Overload (volume/fluid overload refractory to diuretics)
- U - Uremia (asterixis, pericardial rub, encephalopathy, uremic bleeding)
Many nephrologists also initiate dialysis empirically when BUN exceeds ~100 mg/dL without signs of kidney recovery.
RRT modalities:
- Intermittent Hemodialysis (IHD) - standard, for hemodynamically stable patients
- Continuous Renal Replacement Therapy (CRRT) - preferred in septic shock or hemodynamic instability, as it avoids rapid intravascular fluid shifts
- Peritoneal Dialysis - alternative when vascular access is unavailable
"Initiating early RRT has not been demonstrated to improve survival and may increase the risk of adverse events." - Harrison's Principles of Internal Medicine, 22nd Ed.
Prognosis and Long-Term Outcomes
AKI carries significant long-term consequences even after apparent recovery:
- 72.1% of patients who had AKI without pre-existing CKD developed CKD within 2 years
- AKI requiring dialysis is associated with a 3.23× increased incidence of chronic dialysis vs. matched controls
- AKI increases risk of ESKD by 13-fold (no prior CKD) and 41-fold (with prior CKD)
- AKI that required dialysis carries >double the risk of death in the long-term
Recent systematic review (PMID: 40424522) confirms the significant long-term risk of adverse kidney outcomes after AKI episodes.
- Brenner & Rector's The Kidney
Key Concepts to Remember
| Concept | Detail |
|---|---|
| Baseline creatinine | Always needed for AKI diagnosis; if unavailable, KDIGO recommends back-calculating from MDRD assuming eGFR of 75 mL/min/1.73m² |
| AKD (Acute Kidney Disease) | New KDIGO term for kidney dysfunction lasting 7 days to 3 months - between AKI and CKD |
| Anuria vs. Oliguria | Anuria (<100 mL/day) suggests complete obstruction or cortical necrosis; oliguria <400 mL/day |
| "AKI on CKD" | Common scenario - these patients are at highest risk for rapid progression to ESKD |
Sources:
- Campbell-Walsh-Wein Urology
- Brenner & Rector's The Kidney, 2-Volume Set
- Harrison's Principles of Internal Medicine, 22nd Edition (2025)
- National Kidney Foundation Primer on Kidney Diseases, 8th Edition
- Tintinalli's Emergency Medicine
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