Fever with Abdominal Pain in Children: Diagnostic Approach

A. SURGICAL EMERGENCIES

1. Acute Appendicitis

• WBC >10,000 with neutrophilia
• US: non-compressible tubular structure >6mm
• CRP >10mg/L
• Alvarado score >7
• CT: wall thickening, fat stranding

2. Intussusception

• US: target/doughnut sign
• Age 6mo-3yr typical
• Sausage-shaped mass RUQ/epigastric
• Currant jelly stools (late)
• Plain film: absence of cecal gas

3. Malrotation with Volvulus

• UGI: corkscrew sign/abnormal duodenal position
• US: whirlpool sign of SMA
• X-ray: double bubble sign
• Elevated lactate
• Metabolic acidosis

B. INFECTIOUS/INFLAMMATORY CONDITIONS

1. Acute Gastroenteritis

• Stool WBC >5-10/hpf (bacterial)
• Stool culture/PCR panel
• Na+ <135 or >145 mEq/L
• BUN/Cr ratio >20:1 (dehydration)
• Rotavirus/adenovirus antigen test

2. Mesenteric Adenitis

• US: cluster of ≥3 nodes >8mm
• Elevated throat swab/viral PCR
• Normal appendix on imaging
• CRP may be elevated
• Associated URI symptoms

3. UTI/Pyelonephritis

• UA: WBC >5/hpf, positive nitrite/LE
• Urine culture >100K CFU/mL
• US: hydronephrosis/perinephric stranding
• DMSA: cortical defects
• PCT >0.5 ng/mL suggests pyelonephritis

C. HEPATOBILIARY/PANCREATIC DISORDERS

1. Acute Pancreatitis

• Lipase >3x upper limit normal
• US: pancreatic edema/fluid
• ALT/AST may be elevated
• Ca2+ <8.5 mg/dL
• CT severity score if needed

2. Cholecystitis

• US: wall >3mm, pericholecystic fluid
• Murphy sign positive
• ALT/AST/GGT elevation
• Direct bilirubin elevation
• HIDA scan: non-filling GB

D. SYSTEMIC/MISCELLANEOUS

1. HSP (IgA Vasculitis)

• Elevated IgA levels
• Skin biopsy: leukocytoclastic vasculitis
• Urinalysis: proteinuria/hematuria
• Normal platelets (vs ITP)
• Positive stool guaiac (GI involvement)

2. Kawasaki Disease

• ↑ESR/CRP
• Thrombocytosis >450K after day 7
• Sterile pyuria
• ALT >50
• Echo: coronary changes

E. RESPIRATORY CONDITIONS

1. Lower Lobe Pneumonia

• CXR: infiltrate/consolidation
• ↓breath sounds lower zones
• SpO2 <95%
• PCT >0.5 ng/mL (bacterial)
• Positive respiratory PCR panel

2. Pleural Effusion/Empyema

• CXR: fluid level/blunting
• US: septations/loculations
• Pleural fluid: pH <7.2 (empyema)
• Pleural glucose <60 mg/dL
• Positive pleural culture

F. HEMATOLOGIC/ONCOLOGIC CONDITIONS

1. Acute Leukemia

• CBC: blasts, cytopenias
• LDH >500
• Uric acid elevation
• Flow cytometry: blast markers
• Bone marrow: >20% blasts

2. Lymphoma

• CT: lymphadenopathy/masses
• ↑LDH, ↑uric acid
• ESR >50
• Node biopsy: diagnostic
• PET scan: staging

3. Sickle Cell Crisis

• Hemoglobin SS on electrophoresis
• ↓Hb from baseline
• ↑Reticulocyte count
• ↑Bilirubin, ↑LDH
• US: splenic infarction

G. GENITOURINARY CONDITIONS

1. Ovarian Torsion

• US: enlarged ovary >4cm
• Doppler: reduced/absent flow
• Elevated WBC common
• ↑β-hCG if pregnant
• CT: whirlpool sign

2. Testicular Torsion

• US Doppler: absent flow
• High-riding testis
• Loss of cremasteric reflex
• Nuclear scan: photopenic area
• Urinalysis normal

3. Nephrolithiasis

• CT: stone(s) >3mm
• US: hydronephrosis, stone
• UA: hematuria, crystals
• ↑Ca, ↑uric acid, ↑oxalate
• XR: radio-opaque stones

H. RHEUMATOLOGIC CONDITIONS

1. Systemic JIA

• Ferritin >1000
• ESR/CRP markedly elevated
• Quotidian fever pattern
• ANA often negative
• ↑IL-6, ↑IL-1β

2. PAN (Polyarteritis Nodosa)

• ↑ESR/CRP
• ANCA typically negative
• Angiography: microaneurysms
• Tissue biopsy: vasculitis
• Hepatitis B serology

I. METABOLIC/ENDOCRINE CONDITIONS

1. Diabetic Ketoacidosis

• Glucose >250 mg/dL
• pH <7.3, bicarb <15
• Ketones in blood/urine
• Anion gap >12
• Corrected Na+ for calculation

2. Adrenal Crisis

• Na+ <135, K+ >5.0
• Morning cortisol <3 µg/dL
• ACTH >100 pg/mL
• Glucose <60 mg/dL
• Failed ACTH stim test

3. Acute Intermittent Porphyria

• ↑Urinary PBG
• ↑Urinary ALA
• Na+ often <135
• Dark/red urine with light
• Genetic testing: HMBS gene

J. TOXICOLOGIC/INGESTIONS

1. Iron Toxicity

• Serum iron >350 µg/dL
• Abdominal XR: radio-opaque tablets
• Anion gap metabolic acidosis
• ↑AST/ALT after 24hrs
• Transferrin saturation >45%

2. Lead Poisoning

• Blood lead >5 µg/dL
• Basophilic stippling RBCs
• XR: lead lines
• ↓Hemoglobin/anemia
• ZPP >35 µg/dL

K. RARE BUT IMPORTANT CONDITIONS

1. Mediterranean Fever

• MEFV gene mutation
• Periodic fever pattern
• ↑ESR/CRP during attacks
• Family history positive
• Colchicine response

2. Hemolytic Uremic Syndrome

• Schistocytes on smear
• ↓Haptoglobin, ↑LDH
• ↑Creatinine, BUN
• Thrombocytopenia
• Stool STEC if diarrheal

CLINICAL PEARLS

Important Red Flags

• Bilious vomiting → surgical emergency until proven otherwise
• Fever >5 days with abdominal pain → consider Kawasaki
• Night sweats + weight loss → malignancy workup
• Periodic fever + serositis → autoinflammatory disorder
• Travel history crucial for tropical infections

Key Laboratory Studies

• CBC, CMP, CRP essential baseline
• Urinalysis before antibiotics
• Blood culture if T >39°C
• Lipase if epigastric/radiation
• LDH/uric acid if malignancy suspected

Imaging Guidelines

• US first-line for RLQ pain
• CXR for respiratory symptoms
• CT with contrast if US inconclusive
• MRI for chronic/complex cases
• Nuclear studies for specific indications