NAVLE Urinary · ⏱ 10 min read · 📅 Mar 28, 2026 · by NAVLE Exam Prep Team · 👁 0

Feline Lower Urinary Tract Disease (FLUTD): NAVLE Study Guide

What Is FLUTD?

Feline lower urinary tract disease is not a single diagnosis — it is a clinical syndrome defined by signs originating from the bladder and urethra: stranguria, pollakiuria, hematuria, and periuria (urinating outside the litter box). The underlying cause determines treatment and prognosis, and the NAVLE expects you to know how to work through the differential list quickly based on signalment and history.

The classic NAVLE patient is a young to middle-aged, neutered male indoor cat with acute-onset straining. Your job is to decide: is this cat obstructed or not, and what is the underlying cause?

FLUTD Causes by Category

Cause distribution shifts heavily with age. In cats under 10 years, feline idiopathic cystitis (FIC) dominates. Over 10 years, urolithiasis and bacterial UTI become more relevant.

Cause	Frequency (<10 yr cats)	Key Features	NAVLE Clues
Feline Idiopathic Cystitis (FIC)	55–63%	Sterile inflammation, stress-associated, self-limiting in 5–10 days	Multi-cat household, indoor-only, recent life change, negative culture
Urolithiasis	15–21%	Struvite or calcium oxalate most common; may obstruct	Radiopaque stones on radiograph; crystalluria on UA
Urethral Plug	10–20%	Matrix + crystals; exclusive to male cats; causes complete obstruction	Acute anuria in neutered male, turgid non-expressible bladder
Bacterial UTI	<2–8%	Rare in young cats; more common with predisposing conditions (DM, CKD)	Positive culture; bacteriuria on sediment; older or immunocompromised cat
Anatomic / Neoplastic	<5%	Urethral stricture, TCC; typically older cats with chronic or recurrent signs	Recurrent obstruction, mass lesion on imaging, no response to standard Tx

NAVLE PearlFIC is the most common cause of FLUTD in cats under 10 years. Bacterial UTI is actually rare in young cats — fewer than 2–8% of cases. The NAVLE will try to get you to prescribe antibiotics for a young male cat with sterile FIC. Do not do it. Culture first, treat only if positive.

Obstructed vs. Non-Obstructed: Know the Difference Immediately

This is the highest-stakes clinical decision in FLUTD. A non-obstructed cat with FIC is uncomfortable but not dying. An obstructed cat with a urethral plug is a life-threatening emergency within 24–48 hours due to post-renal azotemia, hyperkalemia, and metabolic acidosis.

Parameter	Non-Obstructed	Obstructed
Bladder	Small, firm, expressible	Large, turgid, non-expressible
Urine production	Present (small frequent voids)	Absent (anuria)
Systemic signs	Usually BAR or mildly uncomfortable	Vomiting, obtundation, hypothermia, bradycardia
Potassium	Normal	Elevated (can be >7–9 mEq/L)
ECG findings	Normal	Tall tented T waves → absent P waves → wide QRS (bradycardia)
Azotemia	Absent or mild	Post-renal azotemia, often severe (Cr >8–15 mg/dL)
Emergency?	No (outpatient management)	Yes — treat hyperkalemia before anesthesia

Classic NAVLE TrapThe obstructed cat is bradycardic, not tachycardic. Severe hyperkalemia causes bradycardia by depolarizing cardiac membranes. If the question gives you a cat in urinary obstruction with HR of 90–100 bpm and absent P waves on ECG — that is a hyperkalemic emergency, and calcium gluconate comes before catheterization.

Managing Urethral Obstruction

The order of operations matters. An unstable, hyperkalemic cat cannot safely be anesthetized for catheter placement until the cardiac toxicity is addressed.

Step 1 — Cardiac stabilization: If ECG shows absent P waves, wide QRS, or tented T waves with bradycardia, give calcium gluconate 10% at 0.5–1.5 mL/kg IV over 5–10 minutes with continuous ECG monitoring. This raises the threshold potential and buys you 20–30 minutes to act.

Step 2 — Lower the potassium: IV fluids dilute serum K+ and correct acidosis. If K+ is severely elevated (>7 mEq/L), add regular insulin 0.25–0.5 U/kg IV + dextrose 50% at 2–4 mL/kg (diluted 1:4). Terbutaline 0.01 mg/kg IM is synergistic via beta-2 receptor-mediated K+ shift. Monitor glucose every 30–60 minutes for 4–6 hours post-insulin.

Step 3 — Relieve the obstruction: Once the patient is hemodynamically stable, sedate lightly and pass a urethral catheter. Flush the urethra with sterile saline. Leave an indwelling catheter for 24–48 hours to allow urethral swelling to resolve.

Step 4 — Watch for post-obstructive diuresis: After catheter removal, urine output often jumps to 6–10 mL/kg/hour. This is expected and results from osmotic diuresis (accumulated urea) plus impaired tubular concentrating ability. Match IV fluids to output and monitor electrolytes every 12–24 hours. Hypokalemia during this phase is common and can be severe.

NAVLE TipPost-obstructive diuresis is the flip of the obstruction itself. The cat goes from anuric and hyperkalemic to polyuric and potentially hypokalemic. Both directions are tested. Normal urine output in cats is 1–2 mL/kg/hr. Output of 6–8 mL/kg/hr after catheter removal = post-obstructive diuresis, not iatrogenic fluid overload.

Feline Idiopathic Cystitis (FIC) — The Non-Obstructive Version

FIC is a sterile, stress-associated inflammatory condition of the bladder. The HPA axis appears dysfunctional in affected cats — they have an exaggerated response to perceived environmental stressors and a deficient counterregulatory response. The bladder wall is a secondary target of this neuroendocrine dysregulation.

The classic signalment is a young to middle-aged, neutered male indoor cat in a multi-cat household. The presentation is usually acute with spontaneous resolution in 5–10 days. Recurrence is the main problem — without addressing the underlying stressors, cats cycle through episodes repeatedly.

Diagnosis is by exclusion. No bacteria on culture, no stones on imaging, no mass. Crystalluria can be present but struvite crystals alone do not mean urolithiasis — many healthy cats have crystalluria without clinical disease.

Long-term management centers on multimodal environmental modification (MEMO):

Increase water intake — wet food, water fountains, multiple water sources
Reduce stress — one litter box per cat plus one extra, multiple feeding stations, vertical space, hiding spots
Pheromone therapy — Feliway diffusers for multi-cat households
Amitriptyline or gabapentin for refractory or severe cases

Antibiotics are not indicated for FIC. The exam will dangle them as an option. Do not take the bait unless you have a positive culture.

Urolith Types: Struvite vs. Calcium Oxalate

These are the two dominant crystal types in feline urolithiasis, and the NAVLE tests both their formation conditions and their management.

Feature	Struvite (MgNH4PO4)	Calcium Oxalate
Urine pH	Alkaline (≥7.0)	Acidic (≤6.5)
Typical age	Younger cats	Middle-aged to older (>7 yr)
Sex predilection	None (or slight female)	Male (neutered)
Radiodensity	Radiopaque	Radiopaque (denser)
Can dissolve with diet?	Yes — acidifying dissolution diet	No — must be removed surgically or by urohydropropulsion
Infection association	Can be secondary to UTI (urease-producing bacteria raise urine pH)	Not infection-associated
Prevention diet	Acidifying, low magnesium, low phosphorus	Increased water intake, avoid excess protein/oxalate restriction

Classic NAVLE TrapOnly struvite dissolves with dietary management. Calcium oxalate does not. A question asking about dietary dissolution for calcium oxalate stones is a trap — the answer is removal, not a special diet. Struvite dissolution diets work by acidifying urine and reducing magnesium and phosphorus.

Perineal Urethrostomy (PU)

PU is a surgical procedure that widens the penile urethra by removing the narrow distal urethra and creating a permanent wider stoma. It does not cure FLUTD — it reduces the risk of recurrent urethral obstruction by eliminating the narrowest part of the male feline urethra.

Indications for PU:

Recurrent urethral obstruction (≥2–3 episodes) despite medical management
Urethral stricture that cannot be relieved by catheterization
Urethral trauma with scarring
Failed catheterization or irreducible plug

Complications include stricture at the stoma site, UTI (increased bacterial access via the wider stoma), and incontinence (rare). PU does not eliminate the risk of stone formation or FIC recurrence — the bladder and proximal urethra still have disease.

NAVLE PearlPU increases UTI risk long-term because the wider stoma reduces the natural urethral defense against ascending bacteria. Cats that undergo PU should have urine cultures done if lower urinary tract signs recur — do not just assume it is FIC again.

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Practice Questions

Test yourself before moving on. Click an answer to reveal the explanation.

Question 1 A 3-year-old castrated male indoor domestic shorthair cat presents with a 2-day history of pollakiuria, stranguria, and hematuria. He lives with two other cats and the family recently moved. Physical examination reveals a small, firm, expressible bladder. Urinalysis shows hematuria, sterile pyuria, and struvite crystalluria. Urine culture is pending. What is the most likely diagnosis?

Question 2 A 5-year-old castrated male domestic shorthair cat presents obtunded with 24 hours of anuria, vomiting, and hypothermia. Physical examination reveals a large, turgid, non-expressible bladder. HR is 96 bpm. ECG shows absent P waves, wide QRS complexes, and tall tented T waves. Serum potassium is 8.8 mEq/L. What is the most critical first intervention?

Question 3 A 4-year-old castrated male cat had a urethral catheter removed 8 hours ago following successful relief of a 24-hour obstruction. The cat is now producing urine at 9 mL/kg/hour. Creatinine is improving (3.2 mg/dL, down from 14.6 mg/dL at admission). Potassium on recheck is 2.9 mEq/L. What is the most important immediate management change?

Question 4 A 3-year-old castrated male cat has experienced three urethral obstructions in the past 8 months. Each episode required hospitalization and urethral catheterization. Environmental modification and dietary changes have been implemented after each episode. The cat has no evidence of urolithiasis on imaging and no urethral stricture. What is the most appropriate next step in management?

Question 5 A 2-year-old spayed female domestic shorthair cat is diagnosed with struvite urolithiasis confirmed by stone analysis after urohydropropulsion. She is currently asymptomatic. Urine culture is negative. What is the most appropriate long-term management strategy to prevent recurrence?