What is the fluid of choice for treating dehydration in a general adult population?

Fluid of Choice for Dehydration

For mild to moderate dehydration, reduced osmolarity oral rehydration solution (ORS) is the first-line therapy, while isotonic crystalloids (normal saline or lactated Ringer's/balanced crystalloids) should be administered intravenously for severe dehydration, shock, or when oral intake fails. 1

Oral Rehydration (Mild to Moderate Dehydration)

Reduced osmolarity ORS is recommended as first-line therapy for mild to moderate dehydration in adults. 1 A well-balanced ORS should contain 65–70 mEq/L sodium and 75–90 mmol/L glucose. 1 The total fluid prescription typically ranges from 2200 to 4000 mL/day depending on ongoing losses. 1

• For mild diarrheal illness, diluted fruit juices, flavored soft drinks, saltine crackers, and broths may suffice. 1
• ORS can be combined with other fluid types or supplemented with IV fluids when oral compliance is suboptimal. 1
• Nasogastric administration of ORS may be considered in patients with moderate dehydration who cannot tolerate oral intake. 1

Intravenous Rehydration (Severe Dehydration)

Initial Resuscitation

Isotonic crystalloids (0.9% normal saline or balanced salt solutions like lactated Ringer's) are the fluids of choice for severe dehydration. 1

• Initial bolus: 15–20 mL/kg/hour (1–1.5 L in average adults) during the first hour in the absence of cardiac compromise. 1
• For patients with sepsis and hemodynamic instability: 20 mL/kg bolus should be given initially. 1
• Continue rapid fluid administration until clinical signs of hypovolemia improve (hypotension, low urine output, impaired mental status). 1

Subsequent Fluid Selection

After initial resuscitation, fluid choice depends on corrected serum sodium: 1

• Normal or elevated corrected sodium: 0.45% NaCl at 4–14 mL/kg/hour 1
• Low corrected sodium: 0.9% NaCl at similar rates 1
• Balanced crystalloids (lactated Ringer's or Plasma-Lyte) are increasingly preferred over normal saline as they likely result in slightly reduced hospital stay and lower risk of hypokalaemia. 2, 3

Monitoring Targets

• Urine output should exceed 0.5 mL/kg/hour. 1
• Fluid administration rate must exceed ongoing losses (urine output + 30–50 mL/hour insensible losses + gastrointestinal losses). 1
• In elderly patients, monitor closely for fluid overload, particularly with cardiac or renal disease. 1

Special Populations

Elderly Patients

Isotonic fluids should be administered to older adults with volume depletion, but with heightened caution for fluid overload. 1

• Oral or enteral routes are preferred when feasible. 1
• Subcutaneous fluid administration (hypodermoclysis) may be considered for mild-to-moderate dehydration without hemodynamic instability. 1
• Fluid and sodium intake should be limited due to higher likelihood of impaired cardiac and renal function. 1

Diabetic Ketoacidosis/Hyperglycemic Crisis

Isotonic saline (0.9% NaCl) is the initial fluid of choice for DKA. 1

• Infuse at 15–20 mL/kg/hour during the first hour. 1
• Subsequent fluid selection depends on corrected serum sodium and electrolyte levels. 1

Critical Pitfalls to Avoid

• Do not use isotonic fluids or ORS for hypotonicity (low-intake dehydration with low osmolality), as these conditions require hypotonic fluids instead. 4
• Avoid rapid fluid resuscitation in mild to moderate hypovolemia—it is unnecessary and potentially harmful. 1
• Do not administer aggressive fluid boluses in elderly patients with renal impairment to prevent acute pulmonary edema. 1
• Monitor for fluid overload in elderly patients, especially those with chronic heart or kidney failure. 1
• Ensure potassium replacement once renal function is confirmed (urine output ≥0.5 mL/kg/hour), as dehydration commonly causes potassium depletion. 1

Evidence Considerations

Recent high-quality evidence suggests balanced crystalloids (lactated Ringer's, Plasma-Lyte) may be preferable to normal saline for most resuscitation scenarios. 2, 3 Balanced solutions likely reduce hospital stay by approximately 0.35 days and decrease the risk of hypokalaemia (RR 0.54) compared to 0.9% saline, while producing higher increases in blood pH and bicarbonate levels. 2 Normal saline can cause hyperchloremic metabolic acidosis and has been associated with impaired renal function in some studies. 3, 5