Kidney Function Tests

Kidney Function Tests: Renal Physiology - Kidney Kickstart

• Nephron: Structural & functional unit; ~1 million/kidney.
• Key processes: Filtration (glomerulus), Reabsorption, Secretion (tubules), Excretion.
• Glomerular Filtration Rate (GFR): Volume of plasma filtered per unit time. Normal: 90-125 mL/min/1.73m².
• Tubular functions: Selective reabsorption (glucose, amino acids), secretion (H+, K+), urine concentration.

⭐ Juxtaglomerular apparatus (JGA) cells secrete renin, initiating RAAS for blood pressure & GFR regulation.

Kidney Function Tests: GFR Markers - The GFR Guardians

• Glomerular Filtration Rate (GFR): Key kidney index. Normal: 90-120 mL/min/1.73m².
• Ideal marker: Inulin (gold standard, impractical).

Table: GFR Markers Comparison

• 📌 Creatinine Can be Confusing: Affected by Creatine (muscle), Certain drugs.

⭐ Serum creatinine is inversely related to GFR; affected by muscle mass, diet (meat), drugs (e.g., trimethoprim, cimetidine block secretion).

Kidney Function Tests: Tubular Function - Tube Tune-Up

• Concentration & Dilution:
  • USG: 1.003-1.030; fixed 1.010 (isosthenuria) = tubular damage.
  • Urine Osmolality: 50-1200 mOsm/kg.
• Acidification:
  • Urine pH: 4.5-8.0. $NH_4Cl$ load test for RTA.
• Proteinuria:
  • Microalbuminuria: 30-300 mg/day (ACR 30-300 mg/g); early diabetic nephropathy.
  • Clinical: >300 mg/day. Nephrotic: >3.5 g/day.
  • Types:

Kidney Function Tests: Urinalysis - Pee Power

• Physical: Color (hydration), clarity (turbidity: cells, crystals).
• Chemical (Dipstick):
  • SG: 1.005-1.030 (concentrating ability).
  • pH: 4.5-8.0 (acid-base).
  • Protein: <150 mg/day. Microalbuminuria: 30-300 mg/day (early diabetic nephropathy).
  • Glucose: If blood glucose >180 mg/dL.
  • Blood: Hematuria (GN, stones, UTI).
  • Leukocyte esterase/Nitrites: UTI.
• Microscopic (Sediment): Cells, crystals, casts.

  Cast Type	Significance
  Hyaline	Normal (low), dehydration, exercise
  RBC	Glomerulonephritis (GN)
  WBC	Pyelonephritis, AIN
  Epithelial (RTE)	Acute Tubular Necrosis (ATN)
  Granular (Muddy)	ATN (ischemic/nephrotoxic)
  Waxy (Broad)	CKD (severe, poor prognosis)
  Fatty	Nephrotic syndrome (oval fat bodies, Maltese cross)

⭐ RBC casts are pathognomonic for glomerulonephritis. ​

Kidney Function Tests: KFTs in Disease - Decoding Dysfunction

Decoding kidney dysfunction involves differentiating Acute Kidney Injury (AKI) types and monitoring Chronic Kidney Disease (CKD) progression. Key AKI parameters help pinpoint causes.

• FeNa: $(U_{Na} \times P_{Cr}) / (P_{Na} \times U_{Cr}) \times 100$

Decoding kidney dysfunction involves differentiating Acute Kidney Injury (AKI) types and monitoring Chronic Kidney Disease (CKD) progression. Key AKI parameters help pinpoint causes.

• FeNa: $(U_{Na} \times P_{Cr}) / (P_{Na} \times U_{Cr}) \times 100$

(mOsm/kg) | > 500 | < 350 | Variable | | $U_{Na}## Kidney Function Tests: KFTs in Disease - Decoding Dysfunction

Decoding kidney dysfunction involves differentiating Acute Kidney Injury (AKI) types and monitoring Chronic Kidney Disease (CKD) progression. Key AKI parameters help pinpoint causes.

• FeNa: $(U_{Na} \times P_{Cr}) / (P_{Na} \times U_{Cr}) \times 100$

(mEq/L) | < 20 | > 40 | > 40 (late) | | FeNa (%) | < 1% | > 2% | > 2% (late) | | BUN/Cr Ratio | > 20:1 | < 15:1 | Variable | | Urine Sediment | Bland, hyaline casts | Granular/cellular casts | Normal, RBCs, WBCs |```mermaid %%{init: {'flowchart': {'htmlLabels': true}}}%% flowchart TD

AKI["🩺 AKI Criteria
• ⬆️ Serum Cr• ⬇️ Urine output"]

Assess["📋 Assess Cause
• Order labs/imaging• Physical exam"]

Pre["🩺 Pre-renal AKI
• FeNa < 1%• Fluid responsive"]

Post["🩺 Post-renal AKI
• Dilated calyces• USG obstruction"]

Intrinsic["🩺 Intrinsic AKI
• FeNa > 2%• Urinary casts"]

AKI --> Assess Assess -->|Fluid resp?| Pre Assess -->|USG obstruct?| Post Assess -->|Neither?| Intrinsic

style AKI fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style Assess fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style Pre fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style Post fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style Intrinsic fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8

> ⭐ In Chronic Kidney Disease (CKD), GFR progressively declines. Stages (1-5) are defined by GFR levels (e.g., Stage 3 GFR **30-59** mL/min/1.73m²), impacting prognosis and management.


## High‑Yield Points - ⚡ Biggest Takeaways

> * **Serum Creatinine**: Key **GFR** indicator; affected by **muscle mass**.
> * **BUN**: Reflects **GFR**, **protein metabolism**; tubular reabsorption occurs.
> * **BUN/Creatinine ratio**: Differentiates **pre-renal**, **renal**, **post-renal azotemia**.
> * **eGFR**: Best overall **GFR** measure; calculated via **CKD-EPI** or **MDRD**.
> * **Urine Albumin-to-Creatinine Ratio (ACR)**: Detects early **kidney damage**, especially in **diabetes**.
> * **Cystatin C**: Alternative **GFR marker**, less influenced by **muscle/diet**.
> * **Fractional Excretion of Sodium (FeNa)**: Differentiates **pre-renal AKI** (<**1%**) from **ATN** (>**2%**).