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Investigation for food poisoning with low blood pressure

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"food poisoning" AND "septic shock" AND investigation

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Investigations for Food Poisoning with Low Blood Pressure (Hypotension/Shock)

Low blood pressure in food poisoning signals a severe, potentially life-threatening presentation - either from significant dehydration/hypovolemia OR from septic shock (systemic infection from an invasive pathogen). This demands a two-track investigation: identify the causative organism AND assess end-organ damage.

1. Bedside / Immediate Assessment

InvestigationRationale
Pulse, BP, RR, temperature, SpO2Quantify severity of shock; fever suggests invasive infection
Capillary refill, skin turgor, mucous membranesAssess degree of dehydration
Urine output (catheter)Monitor renal perfusion; oliguria = inadequate tissue perfusion
ECGHypokalaemia/dyselectrolytaemia can cause arrhythmias
Blood glucose (bedside)Hypoglycemia can accompany sepsis

2. Blood Tests (Priority)

Haematological:
  • Full Blood Count (FBC/CBC) - leukocytosis with neutrophilia suggests bacterial infection; bandemia (left shift) indicates serious infection; thrombocytopaenia may indicate HUS or DIC
  • Peripheral blood smear - if HUS is suspected (microangiopathic haemolytic anaemia)
Biochemistry / Metabolic:
  • Serum electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻) - profuse vomiting and diarrhoea cause hypokalaemia, hyponatraemia, and metabolic acidosis
  • Serum osmolality - assesses dehydration severity
  • Urea and creatinine (renal function) - pre-renal AKI common with hypovolemia; HUS causes intrinsic renal failure (especially with E. coli O157:H7)
  • Liver function tests - hepatotoxic pathogens (e.g. Salmonella typhi); organ dysfunction in sepsis
  • Serum lactate - elevated in septic shock (>2 mmol/L = concern; >4 mmol/L = severe sepsis); key prognostic marker
  • Blood glucose - hypoglycemia in sepsis/multi-organ failure
  • Coagulation screen (PT, aPTT, fibrinogen, D-dimer) - DIC in severe sepsis/Shiga-toxin disease
  • CRP / Procalcitonin - procalcitonin differentiates bacterial sepsis from viral/non-infectious causes; guides antibiotic decisions
Blood Gas:
  • Arterial Blood Gas (ABG) - metabolic acidosis (high anion gap in lactic acidosis), respiratory compensation; guides fluid and ventilation decisions
Microbiology:
  • Blood cultures x2 (before antibiotics) - the most important single investigation if septic shock is suspected; Salmonella, Shigella, Campylobacter, Yersinia, Vibrio can all cause bacteraemia
  • Serum toxin assays - if staphylococcal toxic shock or botulism is suspected

3. Stool Investigations

As noted in Henry's Clinical Diagnosis and Management by Laboratory Methods, stool specimens for short-incubation food poisoning should be evaluated for toxins from S. aureus and Bacillus cereus; specimens from patients with longer incubation should go for routine bacterial culture.
  • Stool culture (routine) - for Salmonella, Shigella, Campylobacter (most common bacterial causes); use appropriate transport media (Cary-Blair) if delay expected
  • Stool microscopy - fecal leukocytes indicate invasive infection (Shigella, Salmonella, Campylobacter); RBCs suggest mucosal invasion or HUS
  • Shiga toxin EIA or NAAT - for E. coli O157:H7/STEC (more sensitive than culture; use sorbitol-MacConkey agar if only culture available)
  • C. difficile toxin assay - if recent antibiotics or hospitalization
  • Stool for ova and parasites - if relevant travel history or prolonged course
  • Stool viral PCR panel (norovirus, rotavirus) - if viral aetiology suspected; ELISA/latex agglutination for rotavirus
  • Cholera toxin EIA or PCR - if cholera suspected (epidemic setting, profuse watery "rice-water" diarrhoea with rapid shock)
  • Vibrio culture - request specifically; not in routine protocol; relevant in seafood exposure

4. Urine

  • Urine specific gravity - <1.015 suggests adequate hydration; elevated in dehydration
  • Urine dipstick + microscopy - haematuria/proteinuria in HUS; exclude UTI as confounding cause
  • Urinary sodium - distinguish pre-renal AKI (UNa <20) from intrinsic renal failure

5. Imaging (Selected Cases)

  • Plain abdominal X-ray - if toxic megacolon or perforation suspected
  • CT abdomen/pelvis - if signs of peritonitis, bowel perforation, or if diagnosis is uncertain (elderly patients may need this to exclude surgical causes such as mesenteric ischaemia, perforation, acute cholecystitis)
  • Chest X-ray - if aspiration or pneumonia suspected; also part of sepsis workup

6. Epidemiological / Public Health Investigation

Per Textbook of Family Medicine: the simultaneous occurrence of two or more cases should prompt notification of local health authorities. Food samples (suspected dishes, leftovers) should be collected in clean sealed glass jars for laboratory analysis. This is mandatory in an outbreak setting.
  • Collect and send suspected food samples for toxin and culture
  • Vomitus/gastric contents - for S. aureus or B. cereus toxin if short incubation (<6 hours)
  • Detailed food history (incubation period guides causative agent):
    • <1 hour: chemical agent
    • 1-6 hours: S. aureus, B. cereus (preformed toxin)
    • 8-16 hours: C. perfringens, B. cereus (diarrhoeal toxin)
    • 16 hours: Salmonella, Campylobacter, Shigella, E. coli, Vibrio, Listeria

7. Summary by Urgency

Immediate (before any treatment): Blood cultures x2, serum lactate, ABG, FBC, U&E, glucose, coagulation screen
Early (within 1-2 hours): CRP/procalcitonin, LFTs, stool culture + microscopy + Shiga toxin, urine output monitoring, stool toxin assays
Directed by clinical picture: CT abdomen (if surgical cause possible), blood smear (HUS), cholera toxin, V. cholerae culture, O&P, C. difficile
Key clinical pearl: The pathogen most likely to cause rapid hypotension with food poisoning includes Vibrio cholerae (massive fluid loss), enterotoxigenic E. coli, Salmonella bacteraemia, and S. aureus toxic shock. The incubation period, clinical features, and food exposure history are your most powerful tools in directing the investigation.
  • Henry's Clinical Diagnosis and Management by Laboratory Methods - Specimen Processing
  • Textbook of Family Medicine 9e - Food Poisoning section
  • Swanson's Family Medicine Review - Gastroenteritis investigations
  • Rosen's Emergency Medicine - Childhood Infectious Diarrhea table
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