# Nephrology Quick-Start Guide: Essential Foundations for Clinical Practice

## Learning Objectives

By the end of this handout, you will be able to: - Describe the functional anatomy of the nephron and explain how different segments contribute to urine formation - Interpret basic kidney function tests (creatinine, eGFR, BUN) and recognize their limitations - Classify acute kidney injury using KDIGO criteria - Understand normal electrolyte homeostasis and recognize common disorders - Apply a systematic approach to volume status assessment - Recognize indications for nephrology consultation

## Part 1: Essential Kidney Anatomy & Physiology

### The Nephron: Your Foundation

Each kidney contains ~1 million nephrons, the functional units that filter blood and regulate fluid/electrolyte balance.

**Key Segments and Functions:**

| Segment | Key Functions | Physiologic Target |
|----|----|----|
| **Glomerulus** | Ultrafiltration; filters 180L/day | Protein barrier maintenance |
| **Proximal Tubule** | Reabsorbs 65% filtered Na+, all glucose, amino acids | Energy-dependent transport |
| **Loop of Henle** | Creates medullary concentration gradient; countercurrent multiplication | Urine concentration |
| **Distal Tubule** | Fine-tunes Na+, K+, Ca2+ balance; responds to thiazides | Electrolyte regulation |
| **Collecting Duct** | Final regulation of Na+, K+, water under hormonal control | ADH and aldosterone response |

**Critical Concept:** Normal kidney functions extend beyond filtration: - **Endocrine:** Erythropoietin (RBC production), vitamin D activation (calcium homeostasis) - **Metabolic:** Gluconeogenesis, acid-base balance through ammonia production - **Regulatory:** Long-term blood pressure control through RAAS activation

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## Part 2: Interpreting Kidney Function Tests

### The “Imperfect Trinity” — What You Need to Know

**Serum Creatinine (Scr)** - Normal range: Men 0.7-1.3 mg/dL; Women 0.6-1.1 mg/dL - Produced at relatively constant rate from muscle metabolism - **Limitations:** Influenced by muscle mass, age, sex, medications (trimethoprim, cimetidine can falsely elevate) - Rule of thumb: A “normal” creatinine of 1.2 in a 75-year-old woman may represent significant kidney dysfunction

**Estimated Glomerular Filtration Rate (eGFR)** - Calculated from creatinine using CKD-EPI equation (preferred) - Accounts for age, sex, race - Normal: \>90 mL/min/1.73m² - **Caveat:** Less reliable during acute kidney injury, extremes of body composition, rapidly changing function

**Blood Urea Nitrogen (BUN)** - Normal: 7-20 mg/dL - Influenced by protein intake, liver function, catabolism - BUN:Cr ratio helps differentiate prerenal azotemia (\>20:1) from intrinsic kidney disease (10-15:1)

**Alternative Markers:** - **Cystatin C:** Less influenced by muscle mass; useful in elderly, children - **Spot protein-to-creatinine ratio (SPCR):** 1.0 ≈ 1 g protein/day - **Albumin-to-creatinine ratio:** \<30 mg/g normal; 30-300 mg/g = microalbuminuria

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## Part 3: Urinalysis — The Window into Kidney Disease

### Quick Interpretation Framework

**Physical Properties:** - **Color:** Pale yellow to amber normal; brown/red = hematuria or pigmenturia - **Clarity:** Clear to slightly hazy; cloudy = infection, crystals, or proteinuria - **Specific Gravity:** 1.003-1.030; fixed at 1.010 = concentrating defect

**Chemical Components Interpretation:**

| Finding | Significance | Next Step |
|----|----|----|
| **Protein 1+ or more** | Glomerular disease likely if quantified \>150 mg/day | Quantify with SPCR or 24h urine |
| **Blood positive, RBCs seen** | Hematuria; dysmorphic cells = glomerular; uniform = lower urinary tract | Dysmorphic RBCs + casts = GN |
| **Blood positive, NO RBCs** | Hemoglobinuria or myoglobinuria (intravascular hemolysis or rhabdo) | Check rhabdo labs, LDH, urinalysis |
| **Glucose (no diabetes)** | Proximal tubule dysfunction or very high serum glucose | Check serum glucose; consider Fanconi |
| **Ketones + Glucose** | Diabetic ketoacidosis until proven otherwise | Stat VBG, electrolytes, insulin |
| **Nitrites positive** | Bacterial infection (gram-negatives especially) | Urine culture; empiric abx if symptomatic |
| **Leukocyte esterase** | WBC present; suggests infection or inflammation | Culture; clinical context matters |

**Casts — Diagnostic Clues:** - **RBC casts:** Strongly support glomerular bleeding; urgent evaluation when rapidly progressive disease is suspected - **WBC casts:** Suggest pyelonephritis or interstitial nephritis - **Granular casts:** Coarse = acute tubular necrosis; fine = chronic processes - **Hyaline casts:** May be normal after exercise; increased numbers suggest reduced flow

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## Part 4: Acute Kidney Injury (AKI) — Diagnosis & Classification

### KDIGO Staging System

**Definition:** Serum creatinine rises ≥0.3 mg/dL within 48 hours or ≥1.5 times baseline within 7 days, or urine output is \<0.5 mL/kg/hour for 6 hours; interpret the baseline and clinical context

| Stage | Serum Creatinine | Urine Output | Clinical Significance |
|----|----|----|----|
| **Stage 1** | 1.5-1.9x baseline OR ≥0.3 mg/dL increase | \<0.5 mL/kg/hr × 6-12 hrs | Mild; still reversible |
| **Stage 2** | 2-2.9x baseline | \<0.5 mL/kg/hr × 12-24 hrs | Moderate; requires intervention |
| **Stage 3** | ≥3x baseline OR ≥4.0 mg/dL | \<0.3 mL/kg/hr × 24 hrs OR anuria ≥12 hrs | Severe; may need RRT |

### AKI Etiologies — The Prerenal-Intrinsic-Postrenal Framework

**Prerenal AKI** (55-60% of cases) - Mechanism: Decreased renal perfusion - Key findings: FENa \<1%, Scr/BUN ratio \>20:1, oliguria - Common causes: Hypovolemia, shock, medications (ACEi/ARB in specific settings) - Response to IV fluids: Responsive

**Intrinsic AKI** (35-40% of cases) - Acute Tubular Necrosis (ATN): Ischemic or nephrotoxic; most common intrinsic cause - Acute Interstitial Nephritis (AIN): Drug-induced (NSAIDs, antibiotics, PPIs); eosinophiluria suggests drug reaction - Glomerulonephritis: RBC casts or acanthocytes strongly support glomerular bleeding; establish the cause with clinical evaluation and, when indicated, biopsy - Vascular: Renal infarction, thrombosis, dissection

**Postrenal AKI** (5% of cases) - Mechanism: Urinary obstruction - Findings: Hydronephrosis on ultrasound - Common causes: Stones, BPH, malignancy, strictures - Response: Urgent decompression

### Key Decision Tool: FENa Calculation

**Fractional Excretion of Sodium (FENa) = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100**

- FENa \<1% → Prerenal (conserving sodium)
- FENa \>2% → Intrinsic (wasting sodium)
- **Exception:** High FENa can occur in prerenal if already on diuretics

**Pro Tip:** When AKI is recognized early and prerenal factors identified, aggressive IV hydration can prevent progression to established AKI.

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## Part 5: Electrolyte Disorders at a Glance

### Sodium Disorders

**Hyponatremia (Na \<135 mEq/L)** - Represents water excess relative to sodium - Classify by volume status first (hypovolemic, euvolemic, hypervolemic) - Acute vs chronic: Acute (\<48 hrs) carries seizure risk; chronic adaptation prevents symptoms - **Danger:** Rapid correction causes osmotic demyelination syndrome (ODS) - Correction rate: \<8 mEq/L per 24 hours for chronic hyponatremia

**Hypernatremia (Na \>145 mEq/L)** - Represents water deficit - Causes: Inadequate free water intake, excessive free water losses (DI) - Central DI: Responds to desmopressin; nephrogenic DI: Does not respond - Water replacement must be gradual; too-rapid correction causes cerebral edema

### Potassium Disorders

**Hyperkalemia (K \>5.0 mEq/L)** — THE EMERGENCY - ECG changes correlate poorly with symptom severity - **Emergencies: Peaked T waves → widened QRS → loss of P wave → sine wave → asystole** - Treatment: “ABCs” = Antagonize (calcium gluconate), Shift (insulin/glucose, beta-agonists), Remove (diuretics, dialysis, binders) - Risk factors: eGFR \<15, RAAS inhibitors, NSAIDs, K-sparing diuretics, cell breakdown states

**Hypokalemia (K \<3.5 mEq/L)** - Causes: Diuretics, GI losses, cellular shifts - Cardiac arrhythmias: Increased U waves, flattened T waves, ST depression - **Critical rule:** Every 1 g of KCl deficit ≈ 2-3 mEq/L drop in serum K - Refractory hypokalemia: Check magnesium (need to replete Mg for K to normalize)

### Calcium Disorders

**Hypercalcemia** — Think VITAMINS TRAP - **V**itamins (A, D), **I**ntoxication, **T**umor, **A**ddison, **M**ilk-alkali, **I**mmunity, **N**eoplasm, **S**arcoidosis - **T**hyroid, **R**etinol, **A**ntacids, **P**heo - Presentation: “Stones, bones, groans, and psychiatric overtones” - Severity guide: Mild (\<11 mg/dL) vs Symptomatic (\>13 mg/dL)

**Hypocalcemia** — Think PRIMARY CAUSE - **P**ancreatitis, **R**enal disease (CKD), **I**nfections, **M**alabsorption, **A**lkaline load, **R**Y = Vitamin D deficiency - Check PTH to differentiate PTH-mediated (PTH low) from vitamin D deficiency (PTH elevated) - Acute symptomatic: Calcium gluconate IV (not calcium chloride peripherally — risk of necrosis)

### Magnesium (Often Overlooked!)

- Normal: 1.7-2.2 mg/dL
- **Critical:** ~30% of hypokalemia is magnesium depletion; can’t fix K+ without repleteing Mg
- PPI use is common cause of Mg wasting
- Hypomagnesemia worsens hypokalemia and increases arrhythmia risk

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## Part 6: Volume Status Assessment — The Clinical Skill

### Determining Volume Status (Critical for Treatment Selection)

| Sign/Symptom         | Hypovolemia         | Euvolemia | Hypervolemia            |
|----------------------|---------------------|-----------|-------------------------|
| **Vital Signs**      | Orthostatic changes | Stable    | Hypertension            |
| **JVP**              | Flat (\<2 cm)       | 2-8 cm    | Elevated (\>8 cm)       |
| **Lungs**            | Clear               | Clear     | Rales, pulmonary edema  |
| **Extremities**      | Dry, poor turgor    | Normal    | Peripheral/sacral edema |
| **Mucous Membranes** | Dry                 | Moist     | Normal                  |
| **Urine Output**     | Oliguric            | Normal    | Variable                |

**Physical Exam Pearls:** - Assess JVP with head of bed at 30-45° - Check skin turgor at forearm (not back of hand in elderly) - Orthostatic vital signs: Drop \>20 systolic or \>10 diastolic = significant hypovolemia - Weight changes: 1 L fluid ≈ 1 kg body weight

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## Part 7: Chronic Kidney Disease (CKD) Staging

### KDIGO Staging by eGFR

| Stage  | eGFR  | Description                      | Management Focus          |
|--------|-------|----------------------------------|---------------------------|
| **1**  | ≥90   | Normal/high                      | Treat underlying cause    |
| **2**  | 60-89 | Mildly decreased                 | Slow progression          |
| **3a** | 45-59 | Mildly to moderately decreased   | Control BP, proteinuria   |
| **3b** | 30-44 | Moderately to severely decreased | Prepare for RRT           |
| **4**  | 15-29 | Severely decreased               | Dialysis education begins |
| **5**  | \<15  | Kidney failure                   | RRT or conservative care  |

### Key CKD Complications (Remember: These Accelerate Decline)

1.  **Hypertension:** Needs aggressive control (target SBP \<120 in many)
2.  **Proteinuria:** Every gram/day ≈ faster GFR decline
3.  **Bone disease:** PTH-vitamin D axis dysregulation
4.  **Anemia:** EPO production declines
5.  **Acidosis:** Contributes to bone loss
6.  **Malnutrition:** Wasting affects outcomes

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## Clinical Pearl Summary

**“The Big 5” — Always Consider in Any Kidney Patient:**

1.  **Volume Status** — Hypovolemia looks like “prerenal,” hypervolemia looks like “CHF”
2.  **Medications** — ACEi/ARB, NSAIDs, diuretics, antibiotics are common culprits
3.  **Obstruction** — Never miss a post-renal cause; simple ultrasound is quick
4.  **Infection/Inflammation** — Pyelonephritis, endocarditis present as AKI
5.  **Baseline Renal Function** — You must know the baseline Cr to interpret new values

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## Practice Questions

**Question 1:** A 72-year-old man with baseline creatinine 1.4 mg/dL presents with creatinine 1.9 mg/dL. Which statement is MOST accurate? - A) His eGFR has increased by 5 mL/min/1.73m² - B) He meets KDIGO Stage 1 AKI criteria - C) His actual kidney function loss may be much greater due to lower baseline muscle mass - D) He can safely receive IV contrast without premedication

**Answer:** B is correct (1.9/1.4 = 1.36, which is between 1.5-1.9x baseline = Stage 1). C is a clinical pearl—in elderly patients with lower muscle mass, creatinine changes underestimate kidney function loss.

**Question 2:** Patient with hyponatremia (Na 126), euvolemic, urine osmolality 650 mOsm/kg (high). Most likely diagnosis? - A) Cerebral salt wasting (hypovolemic) - B) SIADH (euvolemic) - C) Nephrogenic DI (hypervolemic) - D) Primary polydipsia

**Answer:** B. Euvolemic + high urine osmolality = kidney appropriately concentrating urine but body can’t handle the water load = SIADH. Treatment: fluid restriction.

**Question 3:** A patient on lisinopril presents with K 6.8 mEq/L and peaked T waves on ECG. Which intervention provides IMMEDIATE cardiac membrane stabilization? - A) IV insulin 10 units with 25 g dextrose - B) IV calcium gluconate 10 mL of 10% solution - C) Sodium polystyrene sulfonate 15 g PO - D) Hemodialysis

**Answer:** B. Calcium antagonizes hyperkalemia’s membrane effects immediately (within seconds). Other treatments lower K+ but take longer. This is the emergency step.

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## Key References

- KDIGO Clinical Practice Guidelines for AKI and CKD (most current)
- Brenner & Rector’s The Kidney, 11th Edition
- National Kidney Foundation Kidney Disease Statistics (annual)
- UpToDate: “Evaluation of acute kidney injury in hospitalized adults”

## Related Student Resources

- nephrology glossary student handout — 100 key terms explained
- structure of renal disease student handout — Framework for organizing renal pathology
- nephrology study topics guide — Curated UpToDate reading list

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*Last updated: February 2026. This handout covers foundational material suitable for PA students, medical students, and early-stage nephrology rotations.*


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