Kidney function evaluation is crucial in diagnosing renal diseases, electrolyte imbalances, acid-base disorders, and systemic conditions like diabetes and hypertension. Below is a detailed clinical discussion covering blood tests, urine tests, imaging, and specialized diagnostics.
I. Blood Tests for Kidney Function Test Purpose Normal Range Clinical Significance Serum Creatinine Measures glomerular filtration 0.6 – 1.2 mg/dL ↑ in AKI, CKD Blood Urea Nitrogen (BUN) Reflects nitrogenous waste excretion 7 – 20 mg/dL ↑ in dehydration, CKD BUN/Creatinine Ratio Differentiates pre-renal vs renal causes 10:1 – 20:1 ↑ in dehydration, ↓ in renal failure eGFR (Estimated GFR) Measures renal function >90 mL/min/1.73m² ↓ in CKD, early renal disease Cystatin C Early kidney dysfunction marker 0.6 – 1.3 mg/L More sensitive than creatinine Electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻) Assess acid-base balance Variable ↑/↓ in AKI, CKD Serum Uric Acid Kidney clearance marker 3.5 – 7.2 mg/dL ↑ in gout, renal disease Serum Calcium & Phosphorus Bone metabolism & kidney disease Ca: 8.5-10.5 mg/dL, Phos: 2.5-4.5 mg/dL Abnormal in CKD PTH (Parathyroid Hormone) Assesses renal bone disease 10 – 55 pg/mL ↑ in CKD (secondary hyperparathyroidism)
II. Urine Tests for Kidney Function 1. Routine Urinalysis (UA)
Appearance : Normal = Clear; Cloudy = InfectionColor : Dark = Hematuria, RhabdomyolysispH : Normal = 4.5 – 8; Alkaline = UTI, RTA; Acidic = Metabolic acidosisSpecific Gravity : 1.005 – 1.030 (High = Dehydration, Low = DI, ATN)Proteinuria : Protein >150 mg/day suggests kidney diseaseGlucosuria : Present in diabetes, Fanconi syndromeKetonuria : Seen in DKA, starvation
2. Microscopic Examination Findings Indication RBCs Hematuria, Glomerulonephritis WBCs UTI, Pyelonephritis Eosinophils Interstitial Nephritis Casts (RBC, WBC, Hyaline, Granular, Fatty) Kidney damage patterns Crystals (Uric Acid, Calcium Oxalate, Cystine, Struvite) Kidney stones
3. 24-Hour Urine Collection
Protein Excretion: >3.5 g/day = Nephrotic syndromeCreatinine Clearance: Estimates GFRSodium, Potassium, Urea Excretion: Determines AKI cause
III. Imaging Studies for Kidney Disease Imaging Purpose Findings Ultrasound (USG-KUB) First-line imaging Hydronephrosis, CKD changes CT KUB (Non-contrast) Detects kidney stones Stones, Obstruction CT Angiography Vascular assessment Renal artery stenosis MRI Renal Soft tissue details Renal tumors, cysts DMSA Scan Assess renal scarring Reflux nephropathy DTPA/ MAG3 Scan Functional assessment Renal perfusion defects
IV. Specialized Kidney Tests 1. Proteinuria Assessment Test Indication Urine Protein/Creatinine Ratio (UPCR) Quantifies proteinuria Albumin-to-Creatinine Ratio (ACR) Detects early kidney damage Electrophoresis Diagnoses Multiple Myeloma (Bence-Jones Proteins)
2. Doppler Ultrasound 🔹 Purpose: Detects Renal artery stenosis, venous thrombosis
3. Arterial Tonometry 🔹 Purpose: Measures arterial stiffness in CKD
V. Functional Tests for Kidney Disease Test Purpose Clinical Use Water Deprivation Test Diagnose Diabetes Insipidus Distinguish central vs nephrogenic DI Furosemide Stress Test Predicts AKI Recovery Low response = worse prognosis Fractional Excretion of Sodium (FENa) Differentiate AKI causes Low in pre-renal, high in ATN Fractional Excretion of Urea (FEUrea) More accurate than FENa Used in patients on diuretics
VI. Kidney Diseases & Differential Diagnosis Disease Type Common Conditions Key Features Infectious UTI, Pyelonephritis Fever, Dysuria, WBC casts Inflammatory Glomerulonephritis, IgA Nephropathy Hematuria, Proteinuria Obstructive Stones, BPH, Hydronephrosis Flank Pain, Dilated Renal Pelvis Functional AKI, CKD Oliguria, Edema Autoimmune Lupus Nephritis, Vasculitis +ANA, +ANCA, Proteinuria
Clinical Decision Flowchart for Kidney Disease Evaluation 📌 Step 1: Assess Symptoms → Edema, Hematuria, Oliguria, Flank PainStep 2: Perform Urinalysis → Proteinuria, Hematuria, CastsStep 3: Measure Serum Creatinine & eGFR → CKD vs AKIStep 4: Imaging (USG, CT KUB) → Structural AbnormalitiesStep 5: Renal Biopsy (If Required) → Glomerular vs Tubular DiseaseStep 6: Monitor and Treat Accordingly
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