STEPTRACKERRENAL | PATHOLOGY + MICROBIOLOGY | HIGH-YIELD REVIEW

Urinary Tract Infection & Obstruction

Bacteria climbing up the urinary tract, the reflux and blockages that help them, and the scars they leave.

FOCUS ON
Cystitis: dysuria, frequency, urgency, no fever; E coli, S saprophyticus. Pyelonephritis: fever, flank pain, CVA tenderness, WBC casts. Chronic pyelonephritis: VUR or obstruction → coarse cortical scars over blunted calyces, "thyroidization." Hydronephrosis: obstruction → dilated pelvis, cortical atrophy.
DISORDER
MECHANISM · ORGANISM
FEATURES
CLUE
Acute cystitis
Ascending infection: E coli (most), S saprophyticus (young sexually active women), Klebsiella, Proteus, Enterococcus
Dysuria, frequency, urgency, suprapubic pain; no fever
⁺ nitrite, leukocyte esterase · culture > 10⁵ CFU
Risk factors
Short female urethra, sexual activity, spermicides, catheters, pregnancy, diabetes, obstruction, VUR
Recurrent infections
Catheters: Enterococcus, Pseudomonas, Candida
Acute pyelonephritis
Ascent to the kidney (P fimbriae of E coli); rarely hematogenous (S aureus)
Fever, chills, flank pain, CVA tenderness, nausea; WBC casts; neutrophils in interstitium and tubules
Mainly cortex · complications: abscess, papillary necrosis
Emphysematous pyelonephritis
Gas-forming bacteria in poorly controlled diabetes
Severe sepsis; gas in renal parenchyma on CT
Emergency
Chronic pyelonephritis
Recurrent infection with VUR (reflux nephropathy) or obstruction
Coarse, asymmetric corticomedullary scars over blunted calyces; tubules with eosinophilic casts ("thyroidization")
Upper and lower poles scarred in reflux
Xanthogranulomatous pyelonephritis
Chronic obstruction + infection (Proteus) with staghorn stones
Destructive mass with foamy macrophages
Mimics renal cell carcinoma
Vesicoureteral reflux
Short intravesical ureter → incompetent valve; primary in children
Recurrent UTIs, pyelonephritis → renal scarring, hypertension, CKD
Voiding cystourethrogram (VCUG)
Hydronephrosis
Obstruction → back-pressure dilates pelvis and calyces → cortical and papillary atrophy
Children: UPJ obstruction, PUV, VUR · adults: stones, BPH, cancers, pregnancy, retroperitoneal fibrosis
Bilateral or single-kidney obstruction → AKI
Retroperitoneal fibrosis
Fibrosis around aorta and ureters: IgG4-related, methysergide, radiation, malignancy
Back pain, bilateral hydronephrosis; ureters pulled medially
Steroids
Asymptomatic bacteriuria
Bacteria in urine without symptoms
Treat only in pregnancy (pyelonephritis, preterm labor) and before urologic procedures
Screen pregnant women
Acute bacterial prostatitis
E coli in older men; STI organisms in young men
Fever, dysuria, perineal pain; tender, boggy prostate
Avoid vigorous prostate massage

Mechanism — ascending infection

Fecal floraColonize periurethra
→
Urethra → bladderType 1 fimbriae
→
Cystitis
→
Reflux / obstructionBacteria reach ureter
→
PyelonephritisP fimbriae bind uroepithelium

Stasis (obstruction, reflux, neurogenic bladder, pregnancy) is the main host factor that lets bacteria ascend.

Organism clues

ORGANISM
CLUE
LAB
E coli
Most common
Nitrite ⁺, lactose fermenter
S saprophyticus
Young sexually active women
Novobiocin-resistant, nitrite ⁻
Proteus
Struvite stones, alkaline urine
Urease ⁺, swarming
Klebsiella
Diabetics, mucoid
Urease ⁺
Enterococcus
Catheters, instrumentation
Nitrite ⁻
Pseudomonas
Hospital, catheters
Oxidase ⁺, blue-green

How it's tested

COMMON CONFUSIONCystitis has no fever and no casts. Fever, flank pain and WBC casts mean the kidney is infected — pyelonephritis.
THE TRAPChronic pyelonephritis has coarse, asymmetric scars with blunted calyces. Hypertensive nephrosclerosis gives fine, symmetric granular shrinkage with normal calyces.
YOUR APPROACHSymptoms (lower vs upper) → host factor (pregnancy, diabetes, catheter, reflux, stone) → organism clue → imaging if recurrent, male, child, or obstructed.

If you see… think…

Young woman with dysuria, no fever, nitrite ⁺ → E coli cystitis
Honeymoon cystitis, nitrite ⁻, novobiocin-resistant → S saprophyticus
Fever, flank pain, CVA tenderness, WBC casts → acute pyelonephritis
Diabetic with sepsis and gas in the kidney → emphysematous pyelonephritis
Child with recurrent UTIs and scarred poles → VUR — VCUG
Tubules full of pink casts resembling thyroid follicles → chronic pyelonephritis
Kidney mass with foamy macrophages and staghorn stone → xanthogranulomatous pyelonephritis
Bilateral hydronephrosis with medially deviated ureters → retroperitoneal fibrosis
Pregnant woman with bacteria but no symptoms → treat asymptomatic bacteriuria
Older man with fever, dysuria and a tender boggy prostate → acute prostatitis