What is the most likely diagnosis for a 25-year-old female with chronic loose stools, intermittent diarrhea, abdominal cramping, and bloating, exacerbated by stress and caffeine, with normal laboratory results, including a normal C-Reactive Protein (CRP) and Complete Blood Count (CBC), and no alarm symptoms?

Most Likely Diagnosis: Irritable Bowel Syndrome with Diarrhea (IBS-D)

This 25-year-old female meets diagnostic criteria for IBS-D based on chronic abdominal pain with altered bowel habits, stress exacerbation, normal inflammatory markers, and absence of alarm features. 1, 2

Diagnostic Reasoning

Positive Symptom-Based Diagnosis

• The patient fulfills Rome criteria for IBS: recurrent abdominal pain for 3 months associated with increased stool frequency (2-3x daily) and altered stool consistency (loose stools/diarrhea), with pain accompanied by bloating and cramping 3, 1
• The symptom pattern of stress-triggered exacerbations and caffeine sensitivity strongly supports IBS, as 60% of IBS patients report stress aggravation of symptoms 3
• The Bristol stool pattern with loose/watery stools occurring more than 25% of the time classifies this as IBS-D subtype 3, 1

Exclusion of Organic Disease

Normal inflammatory markers effectively rule out Crohn's disease:

• CRP of 1.8 (within normal limits) has high negative predictive value for inflammatory bowel disease 1, 2
• Normal CBC excludes anemia, which would be expected in active Crohn's disease 3, 1
• Absence of alarm features (no weight loss, no blood in stool, no fever, no nocturnal symptoms) makes Crohn's disease highly unlikely 3

Celiac disease remains pending but is less likely:

• While anti-tissue transglutaminase antibody is appropriately pending, the clinical presentation lacks typical celiac features such as weight loss, steatorrhea, or malabsorption symptoms 1, 2
• The normal CBC without anemia further reduces celiac probability 3

Key Clinical Features Supporting IBS-D

Behavioral and demographic characteristics:

• Young female (25 years old) - female sex and age <45 years are independent predictors of IBS 4, 2
• Symptoms present for 3 months with stress exacerbation - a hallmark diagnostic behavioral feature 3
• Hyperactive bowel sounds on examination align with IBS-D pathophysiology 3

Absence of alarm features:

• No unintentional weight loss, rectal bleeding, nocturnal symptoms, fever, or family history of IBD/colorectal cancer 3, 1
• Negative fecal occult blood test 3
• Hemodynamically stable with normal vital signs 1

Why Not Crohn's Disease?

Crohn's disease is effectively excluded by:

• Normal CRP - inflammatory bowel disease typically shows elevated inflammatory markers 1, 2
• Normal CBC without anemia or leukocytosis 3, 1
• Absence of weight loss, fever, or nocturnal symptoms 3
• No perianal disease, fistulas, or abdominal masses on examination 3
• Three-month symptom duration without progression or systemic features argues against active inflammation 3

Why Not Celiac Disease?

Celiac disease is less likely because:

• No weight loss or malabsorption symptoms (steatorrhea, fat-soluble vitamin deficiencies) 3, 2
• Normal CBC without iron deficiency anemia, which occurs in 30-50% of celiac patients 3
• While serologic testing is appropriately pending, the clinical picture strongly favors functional disease 1, 2

Diagnostic Approach Validation

The current workup follows evidence-based guidelines:

• Baseline laboratory testing (CBC, CRP, celiac serology) is appropriate for all patients under 45 with these symptoms 1, 2
• Stool PCR pending is reasonable to exclude infectious causes 1
• No further invasive testing (colonoscopy, imaging) is indicated in the absence of alarm features 3, 1, 2

Common Pitfalls to Avoid

Do not over-investigate functional disease:

• Exhaustive testing has low yield and delays appropriate IBS management 5, 6
• Ultrasound or CT imaging uncovers coincidental asymptomatic abnormalities in 8% of cases, potentially leading to inappropriate interventions 3
• Make a positive diagnosis based on symptom criteria rather than continuing exclusionary testing 1, 7, 5

Do not wait for all pending results to initiate management:

• With normal CRP and CBC, the probability of organic disease is extremely low 1, 2
• Patient education, reassurance, and first-line IBS-D treatments (dietary modification, stress management, antispasmodics, loperamide) can begin immediately 2, 5

Clinical Confidence

The diagnosis of IBS-D can be made with high confidence:

• Positive symptom-based criteria are met 3, 1
• Normal inflammatory markers exclude IBD 1, 2
• Absence of alarm features makes serious organic disease unlikely 3
• Young age and female sex increase pre-test probability of IBS 4, 2
• The chance of remaining free of serious disease with this presentation is excellent 3