What causes reddish discoloration of urine?

Causes of Reddish Urine Discoloration

Reddish discoloration of urine is most commonly caused by hematuria (blood in urine), but can also result from hemoglobinuria, myoglobinuria, medications, foods, and metabolic conditions—each requiring distinct diagnostic approaches to differentiate pathologic from benign causes. 1, 2, 3

Pathologic Causes Requiring Urgent Evaluation

Hematuria (True Blood in Urine)

Urologic causes are the most common source of visible blood:

• Malignancy accounts for 30-40% of gross hematuria cases, including bladder cancer (most frequent), renal cell carcinoma, and urothelial carcinoma 1, 4
• Urinary tract infections present with dysuria, frequency, and white blood cells on urinalysis 1, 4
• Urolithiasis (kidney/ureteral stones) typically causes painful hematuria with flank pain 1, 4
• Benign prostatic hyperplasia in older men can cause hematuria but does not exclude concurrent malignancy 1
• Trauma to kidneys or lower urinary tract from injury or instrumentation 1

Glomerular/renal causes present with specific urinary findings:

• IgA nephropathy is the most common glomerular cause of asymptomatic hematuria 1, 4, 2
• Post-infectious glomerulonephritis following streptococcal infection 1, 4
• Alport syndrome presents with hematuria, progressive kidney disease, hearing loss, and ocular abnormalities 1, 4, 2
• Lupus nephritis and vasculitis in patients with systemic autoimmune disease 1

Systemic causes include:

• Coagulopathies (hemophilia) and bleeding disorders 1, 4
• Sickle cell disease causing renal papillary necrosis 1, 4
• Anticoagulants/antiplatelet agents do NOT cause hematuria but may unmask underlying pathology requiring full evaluation 1, 4

Hemoglobinuria and Myoglobinuria

• Hemoglobinuria results from intravascular hemolysis (paroxysmal nocturnal hemoglobinuria, transfusion reactions, severe hemolytic anemia) and appears as cranberry or cola-colored urine 3
• Myoglobinuria occurs with rhabdomyolysis from muscle injury, presenting as tea or cola-colored urine 3, 5
• Both cause positive dipstick for blood but show NO red blood cells on microscopic examination—this is the critical distinguishing feature 3

Benign Causes

Medications and Foods

• Medications causing red/pink urine include rifampin, phenazopyridine, nitrofurantoin, and certain chemotherapy agents 2, 6, 5
• Foods such as beets, blackberries, rhubarb, and food dyes can cause harmless red discoloration 2, 6, 5

Metabolic and Concentration Effects

• Dehydration concentrates urine, making pre-existing microscopic hematuria more apparent, but does NOT cause new RBC entry into urine 7
• Porphyria causes port wine-colored urine that darkens on standing when exposed to light 5

Diagnostic Approach to Differentiate Causes

Initial assessment must distinguish true hematuria from pseudohematuria:

• Dipstick testing detects peroxidase activity and can be falsely positive with myoglobin, hemoglobin, povidone-iodine, and concentrated urine 1, 7
• Microscopic urinalysis showing ≥3 RBCs per high-power field confirms true hematuria and must be performed before any extensive workup 1, 7, 4
• Absence of RBCs on microscopy despite positive dipstick indicates hemoglobinuria or myoglobinuria 3

Distinguish glomerular from non-glomerular bleeding:

• Glomerular bleeding shows >80% dysmorphic RBCs, red blood cell casts (pathognomonic), tea/cola-colored urine, and significant proteinuria 1, 4
• Non-glomerular bleeding shows >80% normal RBCs, bright red urine, and clots 1, 4

Assess urine characteristics:

• Bright red urine suggests lower urinary tract bleeding (bladder, urethra) 1
• Tea or cola-colored urine indicates glomerular disease or hemoglobinuria/myoglobinuria 1, 4, 3
• Pink-tinged urine may represent dilute hematuria or certain medications 1

Critical Clinical Pitfalls

• Never attribute hematuria to anticoagulation alone—these medications unmask underlying pathology that requires full urologic evaluation 1, 4
• Gross hematuria demands urgent urologic referral even if self-limited, given 30-40% malignancy risk 1, 4
• Do not rely on dipstick alone—confirm with microscopic examination showing ≥3 RBCs/HPF before initiating extensive workup 1, 7
• Benign causes (UTI, menstruation, vigorous exercise) do not exclude malignancy in high-risk patients (age >35-40 years, smoking history, occupational chemical exposure) 1, 4
• Hemoglobinuria can be misdiagnosed as hematuria, leading to unnecessary urologic workup when the actual diagnosis is a hematologic disorder like paroxysmal nocturnal hemoglobinuria 3

Risk Stratification for Malignancy

High-risk features requiring complete urologic evaluation:

• Age >60 years in men, >60 years in women 1
• Smoking history >30 pack-years 1, 4
• Occupational exposure to benzenes, aromatic amines, or dyes 1, 4
• History of gross hematuria (even if currently microscopic) 1, 4
• Irritative voiding symptoms without infection 1