Proteinuria - Definition

Normal vs. Pathological Thresholds

• Comprehensive Clinical Nephrology, 7th Edition, p. 64
• Textbook of Family Medicine, 9e, Table 40-7

Mechanisms of Proteinuria

1. Glomerular proteinuria - Disruption of the glomerular filtration barrier (GBM + podocyte slit diaphragm). The barrier is both size-selective (restricts proteins >150 kDa) and charge-selective (glycosaminoglycans repel anionic proteins like albumin). Loss of charge selectivity occurs in minimal change disease; loss of size selectivity occurs in membranous nephropathy.
2. Tubular proteinuria - Impaired reabsorption of low-molecular-weight proteins (albumin, β₂-microglobulin, α₁-microglobulin) by proximal tubular cells, as in Fanconi syndrome.
3. Overflow proteinuria - Systemic overproduction of proteins that overwhelm the tubular reabsorptive capacity (e.g., Bence Jones proteins in multiple myeloma).

Clinical Classification

Clinical Significance

• Proteinuria is both a marker and a pathogenetic factor in progressive renal dysfunction.
• Consistent albuminuria implies glomerular or tubular dysfunction.
• Podocyte injury (foot process effacement) is a cardinal feature of proteinuric renal disease.
• Patients with diabetes or hypertension should be screened periodically (ACR preferred over PCR per KDIGO).
• The ACR (albumin-to-creatinine ratio) on a spot urine sample is the preferred screening tool because albuminuria reliably predicts CKD outcomes.
• Brenner and Rector's The Kidney, 2-Volume Set
• Comprehensive Clinical Nephrology, 7th Edition, p. 64

Pyuria - Definition & Overview

Definition by Method

• Rosen's Emergency Medicine, p. 1391
• Tintinalli's Emergency Medicine

Note: No single accepted level of pyuria is definitively diagnostic of UTI - clinical context is essential.

Significance

• Pyuria with bacteriuria is typical of bacterial UTI (especially coliform organisms).
• Isolated pyuria (without bacteriuria) is unusual, since most inflammatory reactions also cause hematuria.
• WBC casts with bacteria are indicative of pyelonephritis - a key localizing finding.
• WBCs and/or WBC casts also appear in acute glomerulonephritis, interstitial nephritis, and transplant rejection.
• Harrison's Principles of Internal Medicine, 22E (2025)

Sterile Pyuria

• Tuberculosis (classic/important cause)
• Gonorrhea and Chlamydia
• Mycoplasma and Ureaplasma
• Genital herpes
• Trichomoniasis
• Fungal infections
• Schistosomiasis
• Prostatitis, balanitis
• Appendicitis
• Recent antibiotic use (treated UTI within past 2 weeks)

• Interstitial nephritis (e.g., analgesic nephropathy, allergic interstitial nephritis)
• Urinary tract stones
• Polycystic kidney disease
• Recent urinary tract manipulation (catheter, cystoscopy, endoscopy)
• Foreign bodies (surgical mesh, retained stent)
• Atheroembolic disease
• Granulomatous diseases (e.g., sarcoidosis)
• IgG4-related interstitial nephritis
• Tubulointerstitial nephritis and uveitis (TINU) syndrome
• Goldman-Cecil Medicine, Table 100-3

Special Considerations

• Men: >1-2 WBCs/HPF on centrifuged specimen is significant when bacteria are present. In sexually active young males with dysuria, urethritis and prostatitis are far more likely causes than cystitis.
• False-negative pyuria can occur with: dilute urine, systemic leukopenia, self-treatment with antibiotics, or obstructed/infected kidney.
• Eosinophiluria: Requires Hansel stain (more sensitive than Giemsa); classically seen in allergic interstitial nephritis, also in atheroembolic disease, prostatitis, and vasculitis.
• Mononuclear cells in the urine often suggest transplant rejection.
• Tintinalli's Emergency Medicine
• Goldman-Cecil Medicine

Dipstick Testing for Pyuria

• Not all uropathogens produce nitrite (e.g., S. saprophyticus, Enterococcus)
• A negative dipstick does not rule out UTI when clinical suspicion is high - send a urine culture in that setting
• Rosen's Emergency Medicine, p. 1391

Note: Davidson's Principles and Practice of Medicine is not available in the library, so the following is sourced exclusively from Harrison's Principles of Internal Medicine, 22nd Edition (2025).

Pyuria - Harrison's 22E

Definition

Modern data indicate that, at least for older women, median urine WBCs per HPF are far higher in UTI. The cutoff of >10 WBCs/HPF has poor specificity (36%) for UTI, and a cutoff closer to 250 WBCs/HPF might better correlate with presence of urinary symptoms. However, >10 WBCs/HPF remains the generally accepted standard at present.

• Harrison's 22E, Chapter 140 (UTIs)

Key Clinical Associations (Harrison's 22E)

• Typical of bacterial UTI (especially coliform organisms).
• Urine microscopy reveals pyuria in nearly all cases of cystitis and hematuria in approximately 30% of cases.

• Unusual, since inflammatory reactions in the kidney or collecting system are also typically associated with hematuria.
• Presence of bacteria suggests infection.
• WBC casts + bacteria = indicative of pyelonephritis.

• Classic association: urogenital tuberculosis.
• Also seen in: acute glomerulonephritis, interstitial nephritis, and transplant rejection.

• Some patients with inflammatory glomerular disease - such as acute poststreptococcal glomerulonephritis or MPGN - have pyuria from considerable numbers of leukocytes. This must be distinguished from bacterially infected urine.
• Acute nephritic syndrome classically presents with: hypertension, hematuria, RBC casts, pyuria, and mild-to-moderate proteinuria.
• Harrison's 22E, Chapters 55 and 326

Dipstick Testing for Pyuria (Harrison's 22E)

• A dipstick negative for both nitrite and leukocyte esterase should prompt consideration of diagnoses other than UTI.
• A negative dipstick does not rule out bacteriuria in pregnant women - urine culture must be performed for ASB screening in pregnancy.

Pyuria in CAUTI (Harrison's 22E)

• Harrison's 22E, Chapter 140

Haemodialysis - Indications (Harrison's 22E)

A. Indications in Acute Kidney Injury (AKI)

"Dialysis is indicated when medical management fails to control volume overload, hyperkalemia, or acidosis; in some toxic ingestions; and when there are severe complications of uremia (asterixis, pericardial rub or effusion, encephalopathy, uremic bleeding)."

• Harrison's 22E, Chapter 322

• Early initiation of dialysis during sepsis-associated AKI is not associated with improved outcomes - RRT should be started for the definitive indications above.
• Late initiation carries the risk of avoidable volume, electrolyte, and metabolic complications.
• Many nephrologists initiate dialysis empirically when BUN exceeds ~100 mg/dL in patients without signs of renal recovery.
• Initiating dialysis should not await a life-threatening complication.

B. Indications in Chronic Kidney Disease (CKD)

"Encephalopathy and pericarditis are very late complications, so it is now rare that they serve as indications for initiation of renal replacement therapy."

• Harrison's 22E, Chapter 323

C. Timing of Initiation - Harrison's Guidance

• It is ill-advised to assign an arbitrary urea nitrogen or creatinine level to the need to start dialysis - there is individual variability in the severity of uraemic symptoms.
• Patients may become accustomed to chronic uraemia and deny symptoms, only to feel better after dialysis starts.
• Earlier studies supported a "healthy start" concept (starting before severe symptoms), but recent studies have not confirmed that early-start dialysis improves patient survival.
• Practical benefit of early preparation: avoids problems such as a poorly functioning fistula or malfunctioning peritoneal dialysis catheter at the time of urgent need.
• Patient education should begin no later than stage 4 CKD to allow sufficient time for informed decision-making and access preparation.

D. Note on Haemodialysis Technique

• Harrison's 22E, Chapter 322