Dehydration

Overview

🥇 First-line

•U&E - raised urea/creatinine (pre-renal AKI), sodium level (hypernatraemia = hypotonic loss; hyponatraemia = isotonic/hypotonic loss), potassium (GI losses cause hypokalaemia)

•FBC - raised haematocrit/haemoglobin from haemoconcentration

•Urinalysis - concentrated urine (specific gravity >1.020) supports dehydration

•Blood glucose - exclude hyperglycaemia/osmotic diuresis

•Venous blood gas - pH (metabolic acidosis), lactate (poor tissue perfusion), bicarbonate

🏆 Gold standard

•Plasma osmolality - 90% sensitivity, 100% specificity; >295 mOsm/kg confirms hyperosmolar dehydration. Calculated as: 2[Na+] + glucose + urea (mmol/L)

Management

•Restore fluid and electrolyte balance at rate and route appropriate to severity; treat underlying cause in parallel

•Mild-to-moderate: oral rehydration solution (ORS) - preferred route where tolerated

•Severe/haemodynamic compromise: IV fluid resuscitation with 0.9% sodium chloride; add potassium supplementation once urine output established

•Hypernatraemic dehydration: correct sodium by no more than 10-12 mmol/L per 24 hours to avoid cerebral oedema

•Loperamide - adjunct for mild-to-moderate non-bloody, non-febrile diarrhoea; avoid in bloody diarrhoea, suspected dysentery, STEC infection, toxic megacolon risk (C. difficile, ulcerative colitis)

•Antiemetics - to enable oral rehydration where vomiting limits intake

⚠️

Clinical signs (reduced skin turgor, dry mucous membranes) have poor diagnostic accuracy in older patients - these can be present at baseline regardless of hydration status. Always combine clinical assessment with biochemical testing.

Complications

•Pre-renal AKI - reduced renal perfusion; persistent dehydration can progress to intrinsic tubular injury

•Electrolyte disturbances - hypokalaemia (GI losses), hypernatraemia (pure water loss), hyponatraemia (hypotonic loss); risk of arrhythmia and neurological dysfunction

•Hypovolaemic shock - end-stage severe dehydration; cardiovascular collapse

•Delirium - common and reversible precipitant of acute confusion in elderly patients

•VTE - haemoconcentration and immobility increase thrombotic risk

•Over-rapid rehydration - in hypernatraemic states, too-rapid sodium correction risks cerebral oedema