Dehydration
Recognising, treating and preventing fluid deficit — from oral rehydration to IV fluids
Dehydration occurs when the body loses more fluid than it takes in, leading to a deficit of total body water. It is a common and underrecognised condition that ranges from mild thirst and fatigue to severe cardiovascular compromise and acute kidney injury (AKI). Understanding when oral rehydration is adequate — and when IV fluids are needed — is essential for safe clinical practice.
Recognising dehydration
The earliest and most reliable symptom of dehydration is thirst. As dehydration progresses, signs include dry mouth, dark concentrated urine, headache, dizziness, and fatigue. More significant dehydration causes a rapid heart rate (tachycardia), postural hypotension (dizziness on standing), reduced skin turgor, and reduced urine output. Severe dehydration — particularly in elderly patients — can present as confusion or reduced consciousness. The urine colour chart is a practical tool: pale yellow indicates good hydration; dark amber indicates significant fluid deficit. In clinical settings, blood tests showing a raised urea-to-creatinine ratio (>100:1) suggest pre-renal dehydration.
Causes and risk groups
Common causes include inadequate fluid intake (particularly in hot weather, illness, or hospital stay), gastrointestinal losses (diarrhoea and vomiting are the most frequent cause), fever (increasing insensible losses), and diuretic medications (loop and thiazide diuretics promote renal fluid loss). Elderly people are at particular risk because the sensation of thirst diminishes with age — they may become significantly dehydrated before feeling thirsty. Patients with diabetes, kidney disease, or heart failure require careful fluid balance monitoring. Certain medications increase dehydration risk: the 'sick day rules' apply particularly to metformin (lactic acidosis risk), ACE inhibitors and ARBs (AKI risk), NSAIDs (AKI risk), and SGLT-2 inhibitors (DKA risk) — all of which should be withheld during episodes of vomiting, diarrhoea, or poor intake.
Oral rehydration — when and how
Oral rehydration is the first-line treatment for mild to moderate dehydration in people who are able to tolerate fluids. Plain water is adequate for mild dehydration without significant electrolyte losses. However, when there are gastrointestinal losses — diarrhoea and vomiting — oral rehydration salts (ORS) such as Dioralyte are preferred, as they replace both fluid and electrolytes (sodium, potassium, and glucose). Each Dioralyte sachet should be dissolved in exactly 200 mL of water — do not add extra sugar, and do not mix with juice or milk. In adults, 200–400 mL of ORS should replace each episode of loose stool or vomiting. Water alone after significant GI loss carries a risk of hyponatraemia (dangerously low sodium). The target is pale yellow urine, with heart rate and blood pressure returning to normal.
When IV fluids are needed
Moderate to severe dehydration — particularly when the patient is unable to tolerate oral fluids, or when there are signs of haemodynamic instability (heart rate above 100 beats per minute, systolic blood pressure below 100 mmHg) — requires intravenous fluid replacement. The first-line IV fluid for resuscitation is Hartmann's solution (compound sodium lactate) or sodium chloride 0.9% (normal saline) — both are isotonic and replace fluid in the extracellular compartment. A fluid challenge of 500 mL over 15 minutes is the standard initial approach for hypovolaemia. For routine IV maintenance (NICE NG29), 25–30 mL per kg per day of water is recommended, with appropriate electrolyte replacement. Special situations require careful adjustment: hypernatraemic dehydration (sodium above 145 mmol/L) must be corrected slowly — no faster than 10 mmol/L per 24 hours — to avoid cerebral oedema. Patients with heart failure or AKI require smaller fluid boluses and frequent reassessment. This article has been prepared following careful evaluation of NICE CG174, NICE NG29, and BNF Chapter 9.2 guidance.