Differential Diagnoses for Abdominal Pain with Low-Grade Fever (99.4°F)
In an adult presenting with abdominal pain and low-grade fever, the differential diagnosis must prioritize life-threatening conditions first, followed by common infectious and inflammatory etiologies, with the understanding that approximately one-third will have appendicitis, one-third will have other documented pathology (most commonly cholecystitis at 9-11%), and one-third will have no diagnosis established. 1, 2
Immediate Life-Threatening Conditions to Rule Out
Acute mesenteric ischemia must be considered first, particularly if pain is severe and out of proportion to physical examination findings, especially in patients with cardiovascular disease history, atrial fibrillation, or recent MI. 2 This condition has mortality that increases with every hour of delay in treatment. 2
Perforated viscus should be suspected if abdominal rigidity or peritoneal signs are present, requiring immediate surgical evaluation. 2
Common Infectious and Inflammatory Causes
Gastrointestinal Tract
Appendicitis represents approximately one-third of emergency department presentations with acute abdominal pain and fever. 1, 2 Right lower quadrant pain with fever is the classic presentation. 1
Acute cholecystitis accounts for 9-11% of acute abdominal pain cases. 2 Right upper quadrant pain with fever suggests hepatobiliary disease. 1
Diverticulitis typically presents with left lower quadrant pain and fever. 1 This is a common cause requiring CT abdomen/pelvis with contrast for diagnosis. 1
Small bowel obstruction occurs in 4-5% of cases and is responsible for approximately 15% of hospital admissions for acute abdominal pain. 3, 2 The triad of abdominal pain, constipation, and vomiting suggests sigmoid volvulus. 1 History of prior abdominal surgery has 85% sensitivity and 78% specificity for predicting adhesive small bowel obstruction. 3, 1
Acute pancreatitis should be considered in the differential, particularly with epigastric pain radiating to the back. 2
Hepatobiliary System
Cholangitis can present with fever and right upper quadrant pain, and CT with IV contrast increased diagnosis by 100% in one prospective study. 3
Liver abscess may present with fever and right upper quadrant pain. 3
Genitourinary Tract
Pyelonephritis can present with flank pain, fever, and abdominal pain. 3
Renal abscess is detectable on rapid MRI protocols with 99% accuracy. 3
Pelvic inflammatory disease should be considered in women of reproductive age; CT diagnosis increased by 280% following imaging in one study. 3
Intra-abdominal Infections
Intra-abdominal abscess (including perinephric, diverticular, or other loculated abscesses) can present with fever and localized abdominal pain. 3 CT can guide percutaneous drainage. 3
Spontaneous bacterial peritonitis in patients with cirrhosis and ascites presents with abdominal pain, fever (temperature >100°F), and tenderness. 3 Ascitic fluid PMN count >250 cells/mm³ confirms diagnosis. 3
Secondary bacterial peritonitis from surgically treatable intra-abdominal sources can masquerade as spontaneous bacterial peritonitis. 3
Pseudomembranous (C. difficile) colitis is frequently encountered and CT findings are present in the colon in 88% of cases. 3
Vascular Causes
- Aortitis is a rare but important cause of febrile abdominal pain in elderly patients, particularly those over 50 years old. 4 It represents a major cause of mortality as it can be complicated by aneurysm and dissection. 4
Gynecologic Causes (Women of Reproductive Age)
Ectopic pregnancy must be ruled out with mandatory β-hCG testing before any imaging. 1, 2
Ovarian torsion can be detected with 86% sensitivity and 100% specificity on MRI. 3
Tubo-ovarian abscess is a common false-positive for appendicitis in surgical series. 5
Malignancy
Colorectal cancer accounts for approximately 60% of large bowel obstructions. 3
Diffuse tumors such as lymphomas or metastases may present with abdominal pain and fever. 3
Critical Clinical Pearls
Pain location is the most valuable starting point: right upper quadrant suggests hepatobiliary disease, right lower quadrant indicates appendicitis, left lower quadrant points to diverticulitis, and diffuse pain requires broader evaluation. 1
In elderly patients (≥65 years), temperature and laboratory screening tests do not differentiate admittable nonsurgical disease from surgical disease. 6 A significant number (13%) of surgical patients present with normal results for all screening tests. 6 Clinical impression is of greater importance than laboratory tests. 6
Tachycardia is the most sensitive early warning sign of surgical complications and should trigger urgent investigation even before other symptoms develop. 1
The combination of fever, tachycardia, and tachypnea predicts serious complications including anastomotic leak, perforation, or sepsis. 1
In bacteremic older persons, fever (≥100°F) is usually present, but 15% may have "afebrile" bacteremia. 3 Predictors include shaking chills, shock, total band neutrophil count ≥1,500 cells/mm³, and lymphocyte count <1,000 cells/mm³. 3
Common Pitfalls to Avoid
Never assume normal laboratory values exclude serious pathology in elderly or immunocompromised patients—proceed to appropriate imaging based on clinical suspicion. 1, 2, 6
Do not delay imaging in clinically deteriorating patients while pursuing additional non-diagnostic tests. 1
Always obtain β-hCG testing in all women of reproductive age before proceeding with any imaging to avoid delayed diagnosis of ectopic pregnancy and unnecessary fetal radiation exposure. 1, 2
Recognize that immunocompromised and neutropenic patients may have masked typical signs of abdominal sepsis, leading to delayed diagnosis and high mortality. 2