What labs, imaging, and medications are recommended for abdominal pain or nausea without diarrhea in primary care?

Evaluation of Abdominal Pain or Nausea Without Diarrhea in Primary Care

For abdominal pain or nausea without diarrhea in primary care, obtain a complete blood count, C-reactive protein or erythrocyte sedimentation rate, coeliac serology, and consider faecal calprotectin if under 45 years old, then treat nausea with ondansetron 8 mg as first-line antiemetic while investigating the underlying cause. 1, 2

Initial Laboratory Testing

All patients presenting with abdominal pain or nausea should have the following blood tests:

• Full blood count to detect anemia, leukocytosis suggesting infection, or thrombocytopenia 1
• C-reactive protein or erythrocyte sedimentation rate to identify inflammatory processes 1
• Coeliac serology to exclude celiac disease as a cause of chronic symptoms 1
• Faecal calprotectin in patients <45 years of age to exclude inflammatory bowel disease 1
• Liver function tests and serum albumin in appropriate clinical contexts 3
• Lactate and blood gas analysis if there is concern for bowel ischemia, particularly if pain is out of proportion to examination findings 3

The British Society of Gastroenterology emphasizes making a positive diagnosis based on symptoms in the absence of alarm symptoms or signs and abnormalities on these simple blood and stool tests. 1

Imaging Considerations

Imaging is NOT routinely required for all patients with abdominal pain or nausea in primary care. However, specific indications warrant imaging:

• Plain abdominal radiograph is the initial test for suspected obstruction, but has limited sensitivity and negative films do NOT exclude mesenteric ischemia or early obstruction 3
• CT abdomen/pelvis with IV contrast is the definitive test when there is diagnostic uncertainty, alarm symptoms, or concern for serious pathology including obstruction, bowel ischemia, or perforation 4, 3
• Abdominal ultrasound has 96% accuracy for detecting gallstones and should be considered if biliary pathology is suspected 5

There is no role for colonoscopy in routine evaluation unless alarm symptoms are present or there are atypical features suggesting microscopic colitis (female sex, age ≥50 years, coexistent autoimmune disease, nocturnal symptoms, weight loss) 1

Critical Alarm Symptoms Requiring Urgent Evaluation

Immediately assess for the following red flags that mandate urgent imaging or surgical consultation:

• Peritoneal signs (rebound tenderness, guarding, rigidity) indicating possible perforation or bowel necrosis 3
• Hemodynamic instability (hypotension, tachycardia, altered mental status) suggesting intra-abdominal hemorrhage or sepsis 4, 3
• Pain out of proportion to examination findings, the hallmark of mesenteric ischemia with 30-90% mortality 4
• Bilious or feculent vomiting indicating mechanical obstruction 3
• Abdominal distension with diminished bowel sounds classic for bowel obstruction 3

Medication Management for Nausea

Ondansetron is the first-line antiemetic for nausea in primary care based on safety and efficacy profile:

• Ondansetron 8 mg orally is as effective as other antiemetics but not associated with sedation or akathisia 2, 6
• Dosing: 8 mg administered initially, with subsequent 8 mg doses every 8-12 hours as needed 2
• Ondansetron is FDA-approved for prevention of nausea and vomiting and has been shown to be first-line in emergency department settings due to favorable safety profile 2, 6

Alternative antiemetics if ondansetron is contraindicated or ineffective:

• Metoclopramide is the only FDA-approved medication specifically for gastroparesis, but carries risk of akathisia and extrapyramidal effects 7, 8
• Prochlorperazine or metoclopramide require monitoring for akathisia that can develop any time over 48 hours post-administration; decreasing infusion rate reduces incidence 6
• Promethazine is more sedating and may be suitable when sedation is desirable, but has potential for vascular damage with IV administration 6

Important caveat: Ondansetron can prolong QT interval, so avoid in patients with congenital long QT syndrome and monitor ECG in patients with electrolyte abnormalities, congestive heart failure, or bradyarrhythmias 2

Pain Management

For abdominal pain without clear etiology:

• Avoid NSAIDs until bleeding sources are excluded, as they can worsen hemorrhage 4
• Acetaminophen is generally safe for mild to moderate pain
• Opioids should be used cautiously as they can mask progressive ileus and gastric distension 2

When to Refer to Secondary Care

Referral to gastroenterology is warranted in the following situations:

• Diagnostic doubt despite initial workup 1
• Symptoms that are severe or refractory to first-line treatments 1
• Alarm symptoms or signs including unintentional weight loss, progressive dysphagia, persistent vomiting, gastrointestinal bleeding, or iron deficiency anemia 1
• Patient request for specialist opinion 1

Common Pitfalls to Avoid

• Do NOT assume all abdominal pain with nausea is functional dyspepsia or gastritis without excluding serious pathology through appropriate testing 5
• Do NOT rely on clinical assessment alone, as misdiagnosis rates range from 34-68% without imaging in acute presentations 4
• Do NOT discharge patients with unexplained symptoms without ensuring adequate follow-up, as delayed presentations of serious pathology can occur 4
• Do NOT use ondansetron to mask progressive ileus in post-surgical patients or those with chemotherapy-induced symptoms, as it does not stimulate peristalsis and should not replace nasogastric decompression when indicated 2
• Absence of peritonitis does NOT exclude bowel ischemia—lactate and blood gas are essential if ischemia is suspected 3

Supportive Care Measures

All patients with nausea and vomiting require:

• IV fluid resuscitation with aggressive crystalloid administration for dehydration from vomiting 3
• Maintain adequate hydration with at least 1.5 L of fluids per day 5
• NPO status and nasogastric tube decompression are mandatory for bilious vomiting or suspected obstruction 3
• Thiamine supplementation if vomiting persists for >2-3 weeks to prevent neurological effects 5