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Azotemia and Urinary Abnormalities

Chapter 55 | Part 2: Cardinal Manifestations and Presentation of Diseases · Part 2 – Cardinal Manifestations & Presentation · Chapter 55

Key Clinical Points

  1. GFR is the primary metric for kidney function; while inulin or iothalamate are used for direct measurement, plasma creatinine (Pcr) is the standard clinical surrogate.
  2. Prerenal azotemia accounts for 40–80% of acute renal failure cases and is typically reversible upon restoration of perfusion.
  3. Acute Tubular Necrosis (ATN) constitutes ~90% of cases of acute intrinsic renal failure; it is characterized by muddy brown granular casts and specific electrolyte profiles.
  4. Postrenal azotemia (<5% of cases) must be ruled out early via ultrasound to identify potential obstruction.
  5. Nephrotic syndrome is defined by proteinuria >3.5 g/24 h per 1.73 m², hypoalbuminemia, and edema.
  6. Urinalysis is critical for distinguishing prerenal azotemia (FeNa <1%, U osm >500) from ATN (FeNa >2%, U osm <350).
  7. Dysmorphic RBCs and RBC casts are key indicators of glomerular involvement in hematuria.
  8. Polyuria (>3 L/m/24 h) is differentiated into solute diuresis, primary polydipsia, or diabetes insipidus (DI).
  9. Cystatin C may be a more sensitive marker for early GFR decline than Pcr but is influenced by sex, DM, smoking, and inflammation.
  10. The use of race-modified eGFR calculations is being phased out to improve equity in treatment access and drug dosing.

DEFINITION & OVERVIEW

• Azotemia: Definition: Retention of nitrogenous waste products (e.g., urea) resulting from a significantly reduced GFR.

• Glomerular Filtration Rate (GFR): Definition: The clearance of inulin or iothalamate in mL/min. While direct measurement is rarely feasible, plasma creatinine (Pcr) serves as the standard surrogate; GFR falls in roughly inverse proportion to Pcr.

• Creatinine Kinetics and Estimation: Standard Formulas: - Cockcroft-Gault: Used for estimating CrCl based on age, weight, and Pcr. - MDRD: Better correlated with true GFR when GFR <60 mL/min per 1.73 m². - CKD-EPI: Generally more accurate than MDRD in mixed populations.

• Cystatin C: Clinical Utility: Produced at a constant rate from all nucleated cells; potentially more sensitive for early GFR decline than Pcr as it is less affected by muscle mass. However, it is influenced by sex, DM, smoking, and inflammation.

EPIDEMIOLOGY

• Race Modifier in eGFR: Status: The use of race as a biological construct in eGFR calculations is under scrutiny. Impact of Removal: - Positive: Improved access to renal transplant waitlisting for Black patients. - Potential Risks: Overdiagnosis of CKD, inaccurate dosing of drugs eliminated by the kidney (e.g., metformin), and reduced access to imaging or living donation among Black patients.

ETIOLOGY & PATHOPHYYSICOLOGY

• Prerenal Azotemia: Prevalence: 40–80% of acute renal failure cases; typically reversible. Etiologies: Decreased circulating volume (hemorrhage, burns, diarrhea, diuretics), volume sequestration (pancreatitis, peritonitis, rhabdomyolysis), or decreased effective arterial volume (cardiogenic shock, sepsis, renal vasoconstriction).

• Intrinsic Renal Disease: Prevalence: Includes ATN (~90% of cases of acute intrinsic renal failure). Etiologies: Ischemic and toxic ATN; drug-induced interstitial nephritis (antibiotics, NSAIDs, PPIs); severe infections; systemic diseases (SLE, sarcoidosis, Sjögren’s syndrome).

• Postrenal Azotemia: Prevalence: <5% of cases; usually reversible. Mechanism: Urinary tract obstruction (urethra, bladder, or bilateral ureteral obstruction).

Clinical Differentiation of Renal Failure

• Prerenal vs. ATN: Distinguished by urinalysis and urine electrolyte composition. - Prerenal: Normal or hyaline/granular casts; high U osm (>500); low FeNa (<1%). - ATN: Muddy brown granular casts, tubular epithelial cells; low U osm (<350); high FeNa (>2%).

CLINICAL FEATURES

• Uremia: Symptoms (e.g., mental status changes, pericarditis) typically develop only when GFR <15 mL/min.

• Syndromes and Findings (Table 55-1): Key Clinical Indicators for Diagnosis: - Acute/Rapidly Progressive Renal Failure: Anuria, oliguria, recent decline in GFR, hematuria, RBC casts, edema, hypertension. - Chronic Renal Failure: Azotemia >3 months, signs of uremia, renal osteodystrophy (hypocalcemia, hyperphosphatemia), small kidneys bilaterally. - Nephrotic Syndrome: Proteinuria >3.5 g/24 h per 1.73 m², hypoalbuminemia, edema, hyperlipidemia. - Urinary Tract Infection/Pyelonephritis: Bacteriuria (>10⁵ cfu/mL), fever, flank tenderness, pyuria, leukocyte casts. - Renal Tubular Defects: Electrolyte disorders, polyuria, nocturia, large kidneys, tubular proteinuria (<1 g/24 h). - Nephrolithiasis: History of stones, renal colic, hematuria, pyuria. - Urinary Tract Obstruction: Azotemia, oliguria, hydronephrosis (ultrasound), large prostate, large kidneys.

DIFFERENTIAL DIAGNOSIS

• Prerenal vs. ATN: Urinalysis: Prerenal sediment is usually normal or has hyaline/granular casts; ATN features muddy brown granular casts and tubular epithelial cells.

• Vascular & Glomerular: - Vascular (e.g., Atheroemboli): Often involves large vessel occlusion; may present with eosinophil-rich inflammation. - Glomerulonephritis: Characterized by RBC casts and significant proteinuria.

DIAGNOSTIC APPROACH

  1. Initial Assessment (Figure 55-1): Perform Urinalysis and Renal Ultrasound.
  2. Obstruction Check: If Hydronephrosis present → Urologic evaluation / Relieve obstruction.
  3. Morphology/Sediment Evaluation:
  4. Small kidneys, thin cortex, bland sediment, isosthenuria, <3.5 ext{ g} protein/24 h → Chronic Renal Failure.

  5. Normal size kidneys, intact parenchyma → Acute Renal Failure.

  6. Evaluate Acute Renal Failure (Urinalysis Branch):

  7. Muddy brown casts or amorphous sediment → Acute Tubular Necrosis (ATN).
  8. RBCs present → Renal artery/vein occlusion → Action: Angiogram.
  9. WBC, casts, or eosinophils → Interstitial nephritis.

  10. Bacteria → Pyelonephritis.

  11. Evaluate Acute Renal Failure (Urinalysis Normal + Oliguria Branch): Perform Urine Electrolyte analysis.

  12. FeNa < 1\% and U ext{ osm} > 500 → Prerenal Azotemia.

  13. FeNa > 2\% and U ext{ osm} < 350 → Acute Tubular Necrosis (ATN).

  14. Hematuria Evaluation (Figure 55-2):

  15. Check for Proteinuria (>500 mg/24 h), Dysmorphic RBCs, or RBC casts.
  16. Yes → Serologic and hematologic evaluation (ANCA, anti-GBM, complement, cryoglobulins, Hep B/C, VDRL, HIV, ASLO).
  17. No → Proceed to infection check.
  18. Check for Pyuria, WBC casts, or Urine eosinophils.
  19. Yes → Urine culture / Urine eosinophils.
  20. No → Hemoglobin electrophoresis, Urine cytology, UA of family members, 24h urinary calcium/uric acid.
  21. Imaging (IVP / Renal ultrasound).
  22. Yes → As indicated: retrograde pyelography or arteriogram, or cyst aspiration.
  23. No → Cystoscopy.
  24. Cystoscopy.
  25. Yes → Urogenital biopsy and evaluation.
  26. No → Renal CT scan.
  27. Renal CT scan.
  28. Yes → Renal biopsy of mass/lesion.

  29. No → Follow period urinalysis.

  30. Proteinuria Evaluation (Figure 55-3):

  31. Identify proteinuria on dipstick (detects albumin).
  32. Quantify via 24-h collection or protein/creatinine ratio.
  33. Categorize severity:
  34. 30–300 mg/d or mg/g → Moderately increased.
  35. 300–3,500 mg/d or mg/g → Severely increased.

  36. 3,500 mg/d or mg/g → Nephrotic range.

  37. If RBCs or RBC casts present in "Severely" or "Nephrotic" categories → Proceed to Hematuria evaluation (Fig 55-2).

  38. Polyuria Evaluation (Figure 55-4):

  39. Identify polyuria (>3 L/24 h).
  40. Assess Urine Osmolality.
  41. <250 mosmol → Primary polydipsia.

  42. 300 mosmol → Solute diuresis OR Diabetes insipidus (DI).

  43. If DI is suspected (osm >300): Perform Water deprivation test or ADH level.
  44. Vasopressin-sensitive → Central DI.
  45. Vasopressin-insensitive → Nephrogenic DI.

MANAGEMENT & TREATMENT

  1. Address Reversible Causes (Prerenal): Identify and treat underlying causes of hypovolemia, heart failure, or sepsis to restore renal perfusion.

  2. Obstruction Management: Rule out and resolve urinary tract obstruction (urethra, bladder, or ureters) as identified by ultrasound or other imaging.

  3. Chronic Management:

  4. Slow progression of renal disease.

  5. Provide symptomatic relief for edema, acidosis, anemia, and hyperphosphatemia.

  6. Acute Renal Failure Specifics:

  7. Discontinuation of nephrotoxins (e.g., NSAIDs, etc.).

PROGNOSIS & COMPLICATIONS

• Nephrotic Syndrome Complications: Mechanism: Hypoalbuminemia → Reduced plasma oncotic pressure + renal sodium retention → Edema. Compensatory Response: Activation of renin-angiotensin, AVP, and sympathetic nervous system leads to further salt/water reabsorption.

KEY PEARLS & CLINICAL TRAPS

• Prerenal vs. ATN Differentiation: Key Markers: Prerenal (FeNa < 1\%, U ext{ osm} > 500); ATN (FeNa > 2\%, U ext{ osm} < 350).

• Obstruction: Must be ruled out early in any patient with azotemia via ultrasound.

• Nephrotic Syndrome Threshold: Proteinuria >3.5 g/24 h per 1.73 ext{ m}^2 is the defining threshold.

• Polyuria Differentiation: Urine osmolality <250 mosmol suggests primary polydipsia; >300 mosmol suggests solute diuresis or DI.

• Clinical Traps: - A normal urinalysis in a patient with oliguria and azotemia is highly suggestive of prerenal etiology. - Muddy brown granular casts are pathognomonic for ATN. - Presence of eosinophils in urine suggests interstitial nephritis.

Reference Tables

TABLE 55-1 Initial Clinical and Laboratory Database for Defining Major Syndromes in Nephrology SYNDROME Acute or…

Harrison's 22e, p.338

SYNDROME IMPORTANT CLUES TO DIAGNOSIS COMMON FINDINGS CHAP(S). DISCUSSING
DISEASE-CAUSING
SYNDROME
Acute or rapidly progressive renal
failure		 Anuria Hypertension, hematuria 321, 326, 328, 331
Oliguria Proteinuria, pyuria
Documented recent decline in GFR Casts, edema
Hematuria, RBC casts Proteinuria
Azotemia, reduced GFR, oliguria Pyuria
Edema, hypertension Circulatory congestion
Chronic renal failure Azotemia for >3 months Proteinuria, casts 322
Symptoms or signs of uremia, (late
manifestation), casts		 Hypocalcemia, hyperphosphatemia,
hyperparathyroidism
Symptoms or signs of renal osteodystrophy Polyuria, nocturia
Kidneys reduced in size bilaterally Edema, hypertension
Broad casts in urinary sediment Hyperkalemia, metabolic acidosis
Proteinuria, with >3.5 g/24 h per 1.73 m2 Casts
Hypoalbuminemia Lipiduria
Edema Hypercoagulable state
Hyperlipidemia
Asymptomatic urinary abnormalities Hematuria 326
Proteinuria (below nephrotic range)
Sterile pyuria, casts
Bacteriuria, with >105 cfu/mL Hematuria
Other infectious agent documented in urine Mild azotemia and reduced GFR
Pyuria, leukocyte casts Mild proteinuria
Frequency, urgency Fever
Bladder tenderness, flank tenderness
Renal tubular defects Electrolyte disorders Hematuria 327, 328
Polyuria, nocturia “Tubular” proteinuria (<1 g/24 h)
Renal calcification Enuresis
Large kidneys Electrolyte and/or acid-base abnormalities
Renal transport defects Other electrolyte issues, e.g., hypomagnesemia
Systolic/diastolic hypertension Proteinuria
Casts
Azotemia
Nephrolithiasis Previous history of stone passage or removal Hematuria 330
Previous history of stone seen by x-ray Pyuria
Renal colic Frequency, urgency
Azotemia, oliguria, anuria Hematuria
Polyuria, nocturia, urinary retention Pyuria
Slowing of urinary stream Enuresis, dysuria
Large prostate, large kidneys
Flank tenderness, full bladder after voiding

TABLE 55-2 Laboratory Findings in Acute Renal Failure

Harrison's 22e, p.340

INDEX PRERENAL AZOTEMIA OLIGURIC ACUTE
RENAL FAILURE
BUN/P ratio
Cr		 >20:1 10–15:1
<20
Urine osmolality, mosmol/L HO
2		 >500 <350
<1%
Urine/plasma creatinine U /P
Cr Cr		 >40 <20
None or hyaline/granular