Hematuria Proteinuria Polyuria Dialysis Hemodialysis Renal Osteodystrophy Microalbuminuria Diabetic Kidney Disease Acute Tubular Necrosis Drug-induced Renal Disease

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Nephrology: Core Concepts

1. Hematuria

Hematuria is the presence of red blood cells in the urine. It is classified as:

Gross hematuria: visible blood, apparent to patient or clinician
Microscopic hematuria: defined as ≥3 RBCs/high-power field (hpf) in adults; in children, >5 RBCs/hpf on at least two weekly samples is considered abnormal

One prospective study found 61% of patients with hematuria had no significant finding after evaluation. However, the malignancy rate in screening studies is ~2.6%, justifying workup. Most patients with hematuria do not have serious pathology, but this depends on type and risk factors.

Causes include:

Urinary tract infection (cystitis, pyelonephritis)
Kidney stones (nephrolithiasis)
Glomerulonephritis (RBC casts indicate glomerular origin)
Bladder or renal malignancy
Benign prostatic hyperplasia
Trauma, vigorous exercise
Medications (e.g., anticoagulants, cyclophosphamide)
IgA nephropathy (episodic gross hematuria post-infection)

Evaluation depends on whether gross or microscopic, and patient age. Adults with microscopic hematuria require UPr/UCr measurement and evaluation for CKD or urological malignancy.

Textbook of Family Medicine 9e; Brenner and Rector's The Kidney; Campbell Walsh Wein Urology

2. Proteinuria

The kidneys normally excrete only small amounts of protein (mainly glycoproteins) daily. Consistent albuminuria implies glomerular or tubular dysfunction.

Classification by 24-hour urine:

Category	Value
Normal	<30 mg/24 hr
Microalbuminuria	30-300 mg/24 hr
Macroalbuminuria (clinical proteinuria)	>300 mg/24 hr
Nephrotic-range proteinuria	>3.5 g/24 hr

Measurement:

Urine dipstick: convenient but less precise
Urine protein/creatinine (UPr/UCr) ratio: best random-sample test; correlates well with 24-hour excretion; normal adult value <0.2
24-hour urine collections are generally not needed

Mechanisms of proteinuria:

Glomerular injury: podocyte foot process effacement, disruption of the slit pore membrane protein nephrin - the key structural element of the filtration barrier
Angiotensin II dramatically decreases nephrin expression in podocytes; ACE inhibitors restore it
Tubular dysfunction: failure to reabsorb filtered proteins

Types in children:

Functional (fever, exercise) - transient
Orthostatic (benign) - isolated
Persistent proteinuria warrants evaluation for CKD; found on ≥2 of 3 weekly samples

Clinical significance: Proteinuria is not just a marker of kidney disease - it is a pathogenetic factor in the progression of renal dysfunction.

Textbook of Family Medicine 9e; Brenner and Rector's The Kidney

3. Polyuria

Definition: Urine volume >2.5 L/day (conventional); a physiology-based definition considers polyuria present when urine flow is higher than expected for the rate of osmole excretion in a specific setting.

Two categories:

A. Water Diuresis

Caused by failure of vasopressin (ADH) action or production:

Central diabetes insipidus - insufficient ADH release
Nephrogenic diabetes insipidus - renal resistance to ADH
Primary polydipsia - excess water intake suppresses ADH

Key physiology: Water reabsorption depends on aquaporin (AQP) water channels:

AQP1: constitutively present in proximal tubule and descending thin limb (non-regulated)
AQP2: in collecting duct principal cells - regulated by vasopressin; vasopressin causes AQP2 insertion into luminal membrane

Without vasopressin action, urine flow can reach 10-15 mL/min (~14-22 L/day).

B. Osmotic Diuresis

Caused by high concentrations of effective osmoles in the tubular fluid:

Diabetes mellitus (glucosuria)
Mannitol administration
High protein/urea load
Post-obstructive diuresis
Brenner and Rector's The Kidney; Goldman-Cecil Medicine

4. Dialysis

Dialysis is renal replacement therapy (RRT) used when the kidneys can no longer maintain fluid, electrolyte, and waste homeostasis. It is indicated in end-stage kidney disease (ESKD) or severe acute kidney injury.

Two main modalities:

Hemodialysis (HD)
Peritoneal Dialysis (PD)

Choosing between HD and PD: There is a notable absence of comparative studies proving superiority of one modality over another. The choice is based on:

Center experience
Patient and family preference
Socioeconomic circumstances and treatment availability
Compliance capability
Clinical factors (vascular access, peritoneal membrane integrity)

PD advantages:

More liberal fluid intake (important in infants fed liquid nutrition)
Can be done at home
Better school attendance in pediatric patients (74-85% vs 47-59% full schooling)
Often the only option in areas without HD centers

HD considerations:

Requires travel to dialysis center at least 3x/week
In-center HD is time-consuming and costly
In the US, trend has shifted from 34% HD in 1991 to 54% HD in 2010
Brenner and Rector's The Kidney; Comprehensive Clinical Nephrology, 7th Ed.

5. Hemodialysis

Hemodialysis (HD) uses an extracorporeal circuit to filter blood through a semipermeable membrane (dialyzer), removing uremic toxins, excess fluid, and electrolytes.

Vascular Access - three main types:

Access Type	Notes
AV Fistula	Preferred; lowest complication rate; median survival ~3.14 years
AV Graft	Synthetic; higher complication rate than fistula
Central Venous Catheter (CVC)	For short-term or emergent use; median survival ~0.6 year; highest infection and malfunction risk

AV fistula is the recommended choice because it has:

Lower rate of infection and malfunction
Longer access longevity
Better clearance and higher albumin/hemoglobin levels

CVC limitations: Central venous stenosis from CVC use may preclude future ipsilateral AV fistula creation - especially important in younger patients expected to require long-term RRT.

Indications for hemodialysis initiation (in AKI or CKD):

Severe hyperkalemia refractory to medical management
Metabolic acidosis unresponsive to treatment
Pulmonary edema/fluid overload
Uremic encephalopathy, pericarditis, or bleeding
Certain drug/toxin ingestions (e.g., methanol, ethylene glycol, lithium)

Adequacy: Measured by Kt/V (K = dialyzer clearance, t = dialysis time, V = volume of distribution of urea). Target Kt/V ≥1.2 per session for thrice-weekly HD.

Brenner and Rector's The Kidney; Comprehensive Clinical Nephrology, 7th Ed.

6. Renal Osteodystrophy

Renal osteodystrophy (ROD) is the spectrum of bone abnormalities that occur in patients with end-stage renal disease (ESRD).

Pathogenesis:

Metabolic disturbances: hypocalcemia, hyperphosphatemia
Hormonal disturbances: secondary hyperparathyroidism (2° HPT)
Decreased renal synthesis of 1,25(OH)₂D₃ (calcitriol) → decreased intestinal calcium absorption → hypocalcemia → PTH elevation
Phosphate retention (reduced GFR) further suppresses calcitriol and directly stimulates PTH

Two main histologic patterns:

Osteitis Fibrosa Cystica

Due to secondary hyperparathyroidism
Increased bone turnover: accelerated bone dissolution, decreased bone formation
PTH drives osteoclast activity
Brown tumors (fibrous cysts) may form

Osteomalacia

Poor mineralization of bone matrix (osteoid accumulation)
Osteoclasts cannot resorb unmineralized osteoid surfaces - only mineralized surfaces
Osteoclasts must dig "tunneling resorption" through remaining mineralized areas
Historically linked to aluminum toxicity from phosphate binders (aluminum incorporated into hydroxyapatite, blocking further mineralization)

Histologic appearance (Von Kossa stain):

Mineralized bone stains black
Osteoid (unmineralized) stains magenta/red
Thick osteoid seams = osteomalacia
Scalloped osteoclastic resorption = osteitis fibrosa

Patients with ESRD have increased fracture risk - predominantly osteitis fibrosa cystica, osteomalacia, or a combination of both.

Management:

Phosphate binders (calcium-based or non-calcium: sevelamer, lanthanum)
Active vitamin D analogues (calcitriol, paricalcitol)
Calcimimetics (cinacalcet) to suppress PTH
Dietary phosphate restriction

Renal osteodystrophy histology - Von Kossa stain

Renal osteodystrophy: Von Kossa stain showing thick osteoid seams (red/magenta) and osteoclastic tunneling resorption of mineralized bone (black). Arrows indicate cutting cones of osteoclasts. - Henry's Clinical Diagnosis and Management by Laboratory Methods

Henry's Clinical Diagnosis; Robbins Pathology; Grainger & Allison's Diagnostic Radiology

7. Microalbuminuria

Microalbuminuria refers to moderately increased albuminuria: albumin excretion of 30-300 mg/24 hours (or 30-300 mg/g creatinine on spot urine).

Significance:

It is below the detection threshold of standard urine dipstick (which detects protein only when >300 mg/day)
It is a marker of early glomerular damage
A powerful predictor of progressive nephropathy in diabetic patients
Also strongly predicts cardiovascular mortality

In diabetic patients:

Approximately 30% of type 1 diabetics progress to microalbuminuria (stage 2 diabetic nephropathy) after ~10 years
Microalbuminuria and macroalbuminuria significantly increase mortality risk: prospective data from 328 patients with type 2 diabetes showed macroalbuminuria and microalbuminuria both led to markedly elevated 5-year mortality
RAAS blockers (ACE inhibitors, ARBs) can delay onset of microalbuminuria: the ROADMAP trial showed olmesartan produced a 23% delay in time to onset of microalbuminuria in hypertensive patients with type 2 diabetes

Screening: Patients with diabetes should be periodically tested for microalbuminuria. Those with hypertension and other CKD risk factors should also be tested.

Why dipstick misses it: Standard dipstick detects albumin ≥300 mg/day; albumin-specific dipstick or laboratory measurement (ACR) required for microalbuminuria detection.

Goldman-Cecil Medicine; Brenner and Rector's The Kidney; Textbook of Family Medicine 9e

8. Diabetic Kidney Disease (Diabetic Nephropathy)

Diabetic kidney disease (DKD) is the leading cause of ESKD in many countries. In Japan, it overtook glomerulonephritis as the top cause of ESKD in dialysis patients since 1997.

Prevalence: ~30-40% of type 2 diabetic patients develop diabetic nephropathy.

Staging (classical Mogensen stages for type 1):

Stage	Features
I - Hyperfiltration	Elevated GFR, kidney enlargement
II - Silent	Glomerular lesions, normoalbuminuria or microalbuminuria
III - Incipient nephropathy	Fixed microalbuminuria (30-300 mg/day)
IV - Overt nephropathy	Macroalbuminuria (>300 mg/day), declining GFR
V - ESKD	GFR <15, requires renal replacement therapy

Pathogenesis - key mechanisms:

Hemodynamic: Hyperglycemia → afferent arteriolar dilation → increased glomerular capillary pressure (PGC) → hyperfiltration → glomerular hypertrophy
RAAS activation: Despite low plasma renin (paradoxically), prorenin is elevated; the RAS plays a key role in progressive injury
Podocyte injury: Angiotensin II decreases nephrin expression; ACE inhibitor treatment restores nephrin levels
Genetic factors: ACE gene I/D polymorphism - DD genotype associated with higher ACE activity and greater susceptibility to ESKD; IL-10, IL-6, ICAM-1 polymorphisms also implicated
Inflammation: Angiotensin II activates NF-κB → upregulates MCP-1, RANTES, IL-6 → macrophage infiltration

The RAAS paradox in DKD: Despite characteristically low plasma renin in long-standing diabetes, RAAS blockers are the mainstay of therapy - explained partly by elevated prorenin and local (intrarenal) RAAS activation.

Management:

Glycemic control (HbA1c target ~7%)
ACE inhibitors or ARBs: first-line antiproteinuric and renoprotective therapy
SGLT-2 inhibitors (e.g., empagliflozin, dapagliflozin): proven to slow CKD progression
Blood pressure control (<130/80 mmHg)
Dietary protein restriction
Brenner and Rector's The Kidney; Goldman-Cecil Medicine

9. Acute Tubular Necrosis (ATN)

ATN is a clinical syndrome of abrupt, sustained decline in GFR triggered by an acute ischemic or nephrotoxic event, developing within minutes to days after the insult.

Epidemiology: ATN is the most common cause of AKI in hospitalized patients (just under half of all cases); in the ICU, it accounts for more than half of AKI cases.

Causes:

Category	Examples
Ischemic/prerenal spectrum	Cardiogenic shock, septic shock, severe volume depletion, major surgery
Nephrotoxic - exogenous	Aminoglycosides, amphotericin B, cisplatin, radiocontrast agents, NSAIDs
Nephrotoxic - endogenous	Myoglobin (rhabdomyolysis), hemoglobin (hemolysis), uric acid (tumor lysis), myeloma light chains

Pathophysiology:

Obstruction of tubular flow by cellular debris
Disruption of tubular cell polarity and cytoskeleton
Backleak of glomerular filtrate through denuded epithelium into interstitium
Afferent arteriolar vasoconstriction
Loss of microvasculature and immune cell infiltration

ATN has been described as a "(mal)adaptive response" - trading away GFR to preserve medullary oxygenation and tubular integrity. Ischemic ATN is most severe in the outer medulla (proximal straight tubule and thick ascending limb are most vulnerable).

Histology: Vacuolation, loss of brush border, disruption of epithelial lining, intratubular casts. Necrosis is focal and may be missed on biopsy (which samples cortex, not outer medulla well). Regenerative changes and fresh injury often coexist.

Diagnosis:

Finding	Significance
FENa >1-2%	Suggests ATN (intrinsic renal) vs prerenal (<1%)
Fractional excretion of urea >35-50%	Used when diuretics confound FENa
"Muddy brown" granular casts	Hallmark of ATN on urine microscopy; ATN highly likely with ≥6 granular casts
Rising BUN and creatinine	Within days of insult

Clinical course:

Initiation phase (hours to days)
Maintenance/oliguric phase (1-2 weeks typically)
Recovery/diuretic phase

"Muddy brown" granular cast - the hallmark urinary finding of acute tubular necrosis. - Frameworks for Internal Medicine

Frameworks for Internal Medicine; Comprehensive Clinical Nephrology, 7th Ed.; ROSEN's Emergency Medicine

10. Drug-Induced Renal Disease

Drugs can injure the kidney through multiple mechanisms. Major patterns include:

A. Drug-Induced Acute Tubular Necrosis (Nephrotoxicity)

Direct tubular epithelial toxicity:

Aminoglycosides: proximal tubular toxicity (accumulate in lysosomes)
Amphotericin B: direct tubular toxicity + afferent arteriolar constriction
Cisplatin: mitochondrial injury and DNA damage in proximal tubule
Contrast nephropathy: ischemic + toxic mechanisms; risk increases with CKD, diabetes, volume depletion

B. Drug-Induced Acute Interstitial Nephritis (AIN)

Most common cause of AIN; drug use accounts for the majority of cases (particularly antimicrobials, proton pump inhibitors, NSAIDs):

Mean latency from drug exposure to kidney manifestations: ~10 days (can be 1 day to months)
Classic triad: fever (36%), rash (22%), arthralgias (45%) - but often incomplete
Urinary findings: nonnephrotic proteinuria (93%), leukocyturia (82%), microscopic hematuria (67%)
Diagnosis usually requires kidney biopsy

C. Drug-Induced Tubular Secretory Defects (Electrolyte disturbances)

Drug	Mechanism	Effect
NSAIDs/COX-2 inhibitors	Inhibit renin release	Hyperkalemia, metabolic acidosis
ACE inhibitors/ARBs	Block renin-angiotensin	Hypoaldosteronism → hyperkalemia
Heparin	Damages zona glomerulosa; inhibits aldosterone synthase	Hyperkalemia
Spironolactone/eplerenone	Competitive aldosterone inhibitors	Hyperkalemia (especially in renal insufficiency)
Amiloride/triamterene	Block ENaC in collecting duct	Hyperkalemia, reduced K⁺ secretion
Trimethoprim/pentamidine	Structurally similar to amiloride; block ENaC	Hyperkalemia in 20-50% (TMP-SMX); metabolic acidosis
Cyclosporine A/tacrolimus	Inhibit Na⁺-K⁺-ATPase; block ROMK K⁺ channel	Hyperkalemia in transplant recipients

Important note: The combination of potassium-sparing diuretics + ACE inhibitors should be avoided in diabetic patients (who already have hyporeninemic hypoaldosteronism tendency).

D. Lithium Nephrotoxicity

Chronic lithium use causes nephrogenic diabetes insipidus and chronic tubulointerstitial nephritis
Can progress to CKD

E. Calcineurin Inhibitor Nephrotoxicity

Cyclosporine/tacrolimus: cause both acute (afferent arteriolar vasoconstriction) and chronic (fibrosis/striped interstitial fibrosis) nephrotoxicity
Brenner and Rector's The Kidney; Comprehensive Clinical Nephrology, 7th Ed.; Goodman & Gilman's Pharmacology

Quick Reference Summary

Topic	Key Point
Hematuria	≥3 RBCs/hpf (adults); 2.6% malignancy rate in screening
Proteinuria	UPr/UCr ratio best test; >3.5 g/day = nephrotic range
Polyuria	>2.5 L/day; water diuresis (ADH/AQP2) vs osmotic diuresis
Dialysis	HD vs PD - no proven superiority; choice based on patient factors
Hemodialysis	AV fistula preferred access; target Kt/V ≥1.2
Renal Osteodystrophy	Secondary HPT (osteitis fibrosa) + poor mineralization (osteomalacia)
Microalbuminuria	30-300 mg/day; below dipstick threshold; early DKD marker
Diabetic Kidney Disease	Leading cause of ESKD; RAAS blockers + SGLT-2i are cornerstones
ATN	Most common hospital AKI; "muddy brown" casts; FENa >1-2%
Drug-Induced Renal Disease	ATN (aminoglycosides, contrast), AIN (NSAIDs, PPIs), electrolyte defects (TMP, heparin)

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