What initial bloodwork and diagnostic tests would you order for an 18-year-old female presenting with bloody diarrhea for a month?

Diagnostic Workup for Bloody Diarrhea in an 18-Year-Old Female

For an 18-year-old female with bloody diarrhea for one month, the initial bloodwork should include complete blood count, comprehensive metabolic panel, C-reactive protein, erythrocyte sedimentation rate, iron studies, vitamin B12, folate, and thyroid function tests, along with stool testing for infectious pathogens and inflammatory markers. 1

Initial Laboratory Evaluation

Blood Tests

• Complete blood count (CBC) - To assess for anemia, leukocytosis, thrombocytosis/thrombocytopenia
• Comprehensive metabolic panel - To evaluate electrolyte abnormalities, renal function, and liver function
• Inflammatory markers:
  • C-reactive protein (CRP)
  • Erythrocyte sedimentation rate (ESR)
• Nutritional parameters:
  • Iron studies (ferritin, iron, TIBC)
  • Vitamin B12
  • Folate
• Thyroid function tests (TSH, free T4)

Stool Studies

• Stool culture for bacterial pathogens (Salmonella, Shigella, Campylobacter, Yersinia)
• Specific testing for Shiga toxin-producing E. coli (STEC) - Critical in bloody diarrhea
• C. difficile toxin testing - Especially if recent antibiotic use
• Ova and parasite examination
• Stool for inflammatory markers (fecal calprotectin) - To differentiate inflammatory from non-inflammatory causes

Rationale and Considerations

Why These Tests Are Critical

The duration of symptoms (one month) raises concern for both infectious and non-infectious etiologies, particularly inflammatory bowel disease (IBD). The 2003 guidelines for investigation of chronic diarrhea emphasize that screening blood tests have high specificity but low sensitivity for organic disease 1.

Monitoring for Complications

For patients with bloody diarrhea, particularly if STEC is suspected, frequent monitoring of hemoglobin, platelet counts, electrolytes, BUN, and creatinine is recommended to detect early manifestations of hemolytic uremic syndrome (HUS) 1.

Endoscopic Evaluation

Given the prolonged duration (one month), endoscopic evaluation should be strongly considered:

• Flexible sigmoidoscopy - In this age group, most pathology occurs in the distal colon and is accessible with flexible sigmoidoscopy 1
• Colonoscopy with biopsies - May be necessary if sigmoidoscopy is non-diagnostic

Differential Diagnosis Considerations

Infectious Causes

• Bacterial pathogens (Shigella, Salmonella, Campylobacter, STEC)
• C. difficile infection (especially with recent antibiotic exposure)
• Parasitic infections (less common but should be considered)

Non-Infectious Causes

• Inflammatory bowel disease (ulcerative colitis, Crohn's disease)
• Microscopic colitis
• Ischemic colitis (less likely in this age group)
• Medication-induced colitis

Common Pitfalls to Avoid

1. Failing to test for STEC - Critical for preventing complications like HUS
2. Premature empiric antibiotics - May worsen outcomes in STEC infections
3. Overlooking non-infectious causes - In a young female with month-long symptoms, IBD must be considered
4. Inadequate follow-up - If initial testing is negative but symptoms persist, endoscopic evaluation should not be delayed

Additional Considerations

• Celiac disease screening - Consider anti-tissue transglutaminase antibodies, as celiac disease is the most common small bowel enteropathy in Western populations 1
• Laxative abuse screening - Should be considered in cases of factitious diarrhea 1

The chronic nature of this patient's symptoms (one month) places her at higher risk for inflammatory bowel disease, which is particularly important to diagnose promptly in a young patient to prevent complications and optimize quality of life.