Dehydration assessment and rehydration

Dehydration kills approximately 1.7 million people per year globally — almost entirely from diarrheal illness in settings without IV fluid access. The treatment has been known since 1978, when WHO-endorsed oral rehydration salts (ORS) began cutting cholera mortality from 50% to under 1% in field conditions. The formula costs almost nothing to mix from household ingredients. A 60 kg (132 lb) adult with moderate dehydration from gastroenteritis can be fully rehydrated at home with 6–7 liters of correctly formulated ORS over four hours, without a needle or a hospital. The obstacle is almost never supply — it is not knowing the formula, missing the signs early enough, or waiting until the patient is too sick to drink.

Educational use only

This page provides general educational information for emergency preparedness scenarios when professional medical care is unavailable. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified healthcare provider. Use this information at your own risk.

Understanding fluid loss

The body holds roughly 60% of body weight as water — about 42 liters (11 gallons) in a 70 kg (154 lb) adult. Loss of even a small percentage degrades function measurably.

% Body weight lost as fluid	Volume lost (70 kg adult)	Effects
1–2%	700–1,400 mL (24–47 oz)	Thirst, reduced saliva, minor performance decline
3–5%	2,100–3,500 mL (71–118 oz)	Headache, fatigue, dark urine, reduced exercise capacity
6–8%	4,200–5,600 mL (142–189 oz)	Dizziness, rapid heart rate, weakness, skin changes
9–10%	6,300–7,000 mL (213–237 oz)	Confusion, severe weakness, very dark urine or none
Above 10%	Above 7,000 mL (237 oz)	Hypovolemic shock, organ failure, death if untreated

Fluid loss rates by condition

Knowing the source of loss calibrates the replacement rate and formula:

Sweat in heat and exertion: 0.5–2.5 liters/hr (17–85 oz/hr) depending on ambient temperature, humidity, and work intensity. Sweat contains sodium (20–80 mEq/L), potassium, and chloride — electrolyte replacement matters, not just water.
Fever: Each 1°F (0.5°C) above normal increases insensible fluid loss by approximately 10–15%. A person running a 102°F (38.9°C) fever for 24 hours loses an extra 500–700 mL (17–24 oz) beyond normal.
Vomiting: 300–500 mL (10–17 oz) per episode, with significant electrolyte loss.
Diarrhea (cholera-equivalent): Up to 10–20 liters/day in severe cases. This is a medical emergency requiring continuous ORS administration.
Respiratory (breathing): 200–400 mL/day (7–14 oz/day) under normal conditions, significantly more at altitude or in dry cold air.

Severity assessment

Adult dehydration — clinical signs by severity

Mild dehydration (3–5% body weight loss)

Thirst — the earliest reliable indicator in adults under 60
Dry or sticky mouth and lips
Urine that is darker than usual (color 5–6 on the 8-point scale)
Reduced urine frequency (fewer than 3–4 times in 24 hours)
Fatigue and mild headache

Moderate dehydration (6–10% body weight loss)

All mild signs, worsening
Dizziness, especially when standing (orthostatic hypotension — blood pressure drops on standing because volume is insufficient to maintain pressure)
Tachycardia: pulse rate above 100 beats per minute at rest
Skin turgor reduced: pinch the skin on the back of the hand or forearm — it should spring back immediately. In moderate dehydration, the tent of pinched skin takes more than 2 seconds to flatten
Reduced urine output — deeply amber to orange-brown
Muscle cramps

Severe dehydration (above 10% body weight loss)

Confusion, altered mental status — a dehydrated brain cannot function normally
Very rapid, weak pulse (above 120 beats per minute)
Sunken eyes
Skin turgor severely reduced — skin remains tented for 3+ seconds
No urine, or very small amount of dark brown urine
Weak, unable to stand
Cold, pale, or mottled skin
This is hypovolemic shock — treat as an emergency

Severe dehydration is hypovolemic shock

A patient with confusion, rapid weak pulse, and no urine is in pre-shock or frank shock from volume depletion. This cannot be managed with oral fluids alone if the patient cannot swallow or cannot keep fluids down. IV access and evacuation are required. Begin rectal rehydration (proctoclysis) as the bridge while arranging transport — see the proctoclysis procedure below.

Urine color — the 8-point field test

Urine color is the most practical non-equipment field assessment of hydration status. It is validated against urine osmolality as a biomarker of dehydration.

Color number	Description	Hydration status	Action
1	Colorless / very pale yellow	Over-hydrated — excess plain water intake	Reduce intake; add electrolytes
2	Pale straw	Well hydrated	Maintain current intake
3	Pale yellow	Adequately hydrated	Maintain
4	Yellow	Adequate, monitor if exercising or hot	Drink ORS with next exertion
5	Dark yellow	Mild to moderate dehydration	Begin ORS protocol immediately
6	Amber / dark orange	Moderate to severe dehydration	Aggressive ORS — 500 mL/hr
7	Orange-brown	Severe dehydration — possible myoglobinuria	Emergency — ORS + evacuation if not improving
8	Brown / cola-colored	Myoglobinuria or severe dehydration	Emergency — IV fluids required; rule out rhabdomyolysis

Note on color 1: Colorless urine is not a hydration goal. It indicates overhydration with plain water, which dilutes sodium and risks hyponatremia (see the hyponatremia section below). The goal is color 2–4.

Note on color 8: Cola-colored urine may indicate myoglobinuria from muscle breakdown (rhabdomyolysis), not just dehydration. If the patient has had significant exertion or heatstroke, see heatstroke recognition and cooling for rhabdomyolysis management.

Pediatric dehydration

Children dehydrate faster than adults for three reasons: higher body surface area to weight ratio, higher baseline fluid turnover, and inability to self-report or self-manage thirst. Infants and toddlers are most vulnerable. A 10 kg (22 lb) toddler who loses 1 liter to diarrhea has lost 10% of body weight — the severe dehydration threshold — from a volume that would cause only mild symptoms in an adult.

Pediatric signs by severity

Mild (3–5% weight loss) - Slightly less active than normal - Slightly dry mouth - Normal or slightly reduced urination - Thirst if old enough to express it

Moderate (6–10% weight loss) - Decreased skin turgor: the pinch test on the abdomen or thigh remains tented for 2+ seconds - Prolonged capillary refill: press the fingernail until white, release — normal refill in less than 2 seconds; moderate dehydration: 2–3 seconds - Dry mucous membranes: lips, tongue, inside cheek - Tachycardia (age-appropriate rates: infants above 160 bpm, toddlers above 150 bpm, children above 130 bpm) - Irritability or increased crying - Reduced frequency of wet diapers — fewer than 6–8 per day in an infant

Severe (above 10% weight loss) - Sunken anterior fontanelle (the soft spot on top of the skull in infants under 18 months) - Sunken eyes - No tears when crying - Very dry mouth with no saliva pooling - Markedly abnormal mental status: lethargic, difficult to rouse, floppy - Cold extremities, mottled or pale skin - Capillary refill above 3 seconds - Absent or minimal urine output - Weak rapid pulse

Any infant with severe dehydration signs is a medical emergency requiring IV access and evacuation. Use oral rehydration while preparing for transport — do not delay ORS administration while waiting for IV.

Field note

A child who is alert and actively crying — even inconsolably — is probably not severely dehydrated. A lethargic child who is limp and difficult to rouse is. The clinical distinction between "upset" and "obtunded" is the most important pediatric dehydration assessment skill. When in doubt, begin ORS immediately and watch the response over 30 minutes.

The WHO ORS formula

The WHO oral rehydration salts formula was developed from research in the 1960s showing that sodium-glucose co-transport in the gut allows sodium (and water following it osmotically) to be absorbed even when secretory diarrhea is dumping fluid into the intestine faster than the body can absorb it. The current WHO reduced-osmolarity formula (revised 2002) was shown to reduce stool output by approximately 20–30% and decrease the need for IV treatment compared to the original formula.

Official WHO reduced-osmolarity formula (per liter of clean water)

Ingredient	Amount	Field equivalent
Sodium chloride (table salt)	2.6 g	Slightly less than ½ teaspoon (2.5 mL)
Trisodium citrate dihydrate	2.9 g	Approximately ½ teaspoon (2.5 mL)
Potassium chloride	1.5 g	¼ teaspoon (1.25 mL) — available in "no salt" salt substitutes
Glucose (anhydrous)	13.5 g	3 level teaspoons (15 mL) of white sugar
Water	1 liter (34 oz)	Must be previously boiled or treated

Resulting electrolyte composition: Sodium 75 mEq/L, potassium 20 mEq/L, chloride 65 mEq/L, citrate 10 mEq/L, glucose 75 mmol/L, osmolarity 245 mOsm/L.

Simplified field ORS recipe (when trisodium citrate is unavailable)

Trisodium citrate is the buffer component. If unavailable, substitute baking soda (sodium bicarbonate):

1 liter (34 oz) clean water
2.6 g salt (slightly less than ½ teaspoon)
2.5 g baking soda (½ teaspoon)
1.5 g potassium salt (¼ teaspoon "no-salt" substitute) — omit if unavailable, not ideal
13.5 g sugar (3 level teaspoons)

This variant has slightly higher osmolarity but is clinically effective.

Minimum emergency recipe (when only salt and sugar are available)

When potassium and citrate/baking soda are unavailable:

1 liter (34 oz) clean water
1/4 teaspoon (1.5 g) table salt
6 teaspoons (30 g) white sugar

This is the UNICEF "home ORS" recipe. It does not match the WHO formula exactly, but is far superior to plain water for replacing GI losses. It will not replace potassium adequately in severe diarrheal illness; supplement with banana, potato broth, or coconut water if available.

Mixing procedure

Measure water volume first — 1 liter precisely. Use a measuring container, a marked water bottle, or count 33 fluid ounces.
The water must be from a safe source — treated, boiled, or bottled. ORS placed in contaminated water defeats the purpose.
Add each ingredient and stir until fully dissolved. Undissolved salt granules create high-concentration spots.
Do not add more sugar than specified — high sugar concentrations cause osmotic diarrhea, worsening fluid loss.
Consume within 24 hours or discard. Store in a covered container in the coolest available location. ORS does not stay fresh — bacteria will colonize it overnight at warm temperatures.

Oral rehydration procedure

Administration by severity

Mild dehydration — maintenance and replacement

Calculate the replacement volume: 50 mL per kg of body weight for the first 4-hour period.
Example: 60 kg (132 lb) adult = 3,000 mL (3 liters) over 4 hours
Example: 15 kg (33 lb) toddler = 750 mL over 4 hours
Divide the volume into doses: approximately 100–150 mL (3–5 oz) every 20 minutes for adults.
Continue at a maintenance rate after the 4-hour replacement: 10 mL/kg after each loose stool, 2 mL/kg after each vomit episode.

Moderate dehydration

Replacement volume: 100 mL per kg over the first 4 hours.
Example: 60 kg adult = 6,000 mL (6 liters) over 4 hours — approximately 500 mL (17 oz) per 20 minutes
Example: 15 kg toddler = 1,500 mL over 4 hours
If the patient tolerates the intake rate: continue. If vomiting occurs, reduce to small frequent doses (see vomiting protocol below).
Reassess at 4 hours. If improving (alertness better, urine output increasing, skin turgor improving): continue replacement. If not improving: escalate.

Severe dehydration — emergency

Severe dehydration with shock signs cannot be treated with oral fluids alone if the patient cannot drink or keep fluids down. Prioritize: 1. IV access if trained and equipped — normal saline 500 mL–1 liter bolus. 2. Rectal rehydration (proctoclysis) while preparing for transport. 3. Evacuation.

Do not delay ORS administration while waiting for IV — even small amounts of absorbed ORS are beneficial.

Managing vomiting during rehydration

Vomiting is the most common reason oral rehydration fails. The key insight: even a patient who is vomiting absorbs a significant fraction of small-volume doses before vomiting. Give small volumes more frequently.

Give 5 mL (one teaspoon) of ORS every 1–2 minutes using a teaspoon or oral syringe.
Wait 10 minutes after a vomiting episode, then resume at 5 mL doses.
If the patient vomits again, wait 10 minutes and try again. Persistent vomiting despite small-dose administration signals that the gut is not tolerating any volume — escalate to rectal route.
Ondansetron (Zofran) at 4–8 mg orally significantly reduces vomiting frequency in gastroenteritis and dramatically improves ORS tolerance. This is a prescription medication — if stocked in a medical kit, this is its primary preparedness use.

Rectal rehydration (proctoclysis)

Proctoclysis is the rectal infusion of fluids — a technique that bypasses the vomiting patient's upper GI tract entirely. The rectal mucosa absorbs fluid and electrolytes effectively, though more slowly than the small intestine. It was used widely before IV fluids existed, remained in military field medicine through World War I, and is currently used in palliative care and field medicine for patients who cannot tolerate oral or IV routes.

When to use proctoclysis

Indications: - Patient is vomiting continuously and cannot retain even 5 mL doses - Patient is unconscious or unable to swallow - No IV access available and patient has moderate to severe dehydration - Evacuation is hours away and the patient cannot maintain oral intake

Contraindications: - Rectal injury, bleeding, or suspected rectal trauma - Diarrheal illness where the rectum is already inflamed (less effective absorption) - Recent rectal surgery

Equipment

Flexible catheter: a 22 French nasogastric tube cut to length, IV extension tubing, an enema bag tubing set, or improvised irrigation tubing
Lubricant: petroleum jelly, antibiotic ointment, or any food-grade oil
ORS solution (prepared as above) — room temperature to slightly warm, not hot
A container elevated above the patient (gravity-fed drip) or a 60 mL irrigation syringe for bolus delivery

Procedure

Position the patient on their left side (left lateral decubitus) with knees slightly bent toward the chest.
Lubricate the catheter tip generously.
Insert the catheter approximately 4 inches (10 cm) — about the length of an adult finger — past the anal sphincter. Do not force. If resistance is felt, withdraw slightly and redirect.
Connect the catheter to the ORS container elevated 12–18 inches (30–45 cm) above the patient's hips. Gravity feeds the fluid at a controlled rate.
Infusion rate: 250 mL (8 oz) per hour is the standard rate. Faster rates cause cramping and will be expelled.
If the patient complains of rectal cramping or pressure: clamp the tubing and wait 5–10 minutes. The cramping will pass. Resume at a slower rate.
Continue infusion until: the patient can tolerate oral fluids and is resuming oral intake, IV access is established, or the patient reaches a medical facility.
Maximum rate: approximately 1,000 mL (1 liter) per hour — the upper limit before the rectal reflex forces expulsion. This rate requires continuous monitoring.

Total deliverable volume by rectal route: 1–3 liters over several hours in most patients, depending on bowel tone and fluid state. This is sufficient to stabilize mild to moderate dehydration when oral route is unavailable.

Fluid requirements in field conditions

Plain water consumption requirements vary substantially with conditions. These figures assume a healthy adult at rest; all values increase with activity and temperature.

Condition	Daily fluid requirement
Sedentary, temperate (65–70°F / 18–21°C)	2–3 liters (68–102 oz) per day
Light activity, temperate	3–4 liters (102–136 oz) per day
Moderate exertion, warm (80–90°F / 27–32°C)	4–6 liters (136–203 oz) per day
Heavy exertion, hot (above 95°F / 35°C)	6–10 liters (203–338 oz) per day
Fever (each 1°F / 0.5°C above normal)	Add 300–500 mL per 24 hours
Nursing infant or toddler (per kg body weight)	100 mL/kg/day baseline

During heavy exertion in heat, sodium losses through sweat can reach 1–2 g per liter of sweat. Replace with ORS or electrolyte tablets, not plain water, if sweating exceeds 1–2 liters per hour. See heatstroke recognition and cooling for heat illness fluid management and the water storage section for pre-positioning water for extended operations.

Hyponatremia — the danger of overdrinking

Overdrinking plain water during prolonged exertion is a genuine medical emergency that has killed marathon runners and military trainees. It is less common than dehydration but paradoxically more dangerous because the signs mimic severe dehydration and the instinct to give more water makes the condition worse.

Mechanism

When large volumes of plain water are consumed — particularly during prolonged exercise that inhibits the kidney's ability to excrete free water — blood sodium is diluted. The brain swells as water moves into brain cells. Symptoms appear when sodium drops below approximately 130 mEq/L (normal: 135–145 mEq/L).

Who is at risk

Endurance athletes who drink to a schedule rather than thirst
Military personnel required to drink fixed volumes regardless of individual sweat rate
Children given large volumes of plain water during GI illness instead of ORS
Hikers in prolonged hot conditions drinking plain water without salt intake

Signs and symptoms

Early signs (sodium 130–135 mEq/L): - Nausea, headache - Puffiness or bloating despite apparent dehydration - Confusion and disorientation - These signs look like dehydration and are commonly misdiagnosed

Late signs (sodium below 125 mEq/L): - Seizures - Loss of consciousness - Respiratory arrest

The distinguishing sign: A patient with hyponatremia has clear or nearly-clear urine (very well hydrated), bloating, and confusion. A patient with dehydration has dark urine and is dry. A confused patient with clear urine after drinking large volumes of plain water has hyponatremia until proven otherwise — do not give more water.

Management

Stop plain water intake immediately.
Give ORS with sodium — the sodium is the treatment. Commercial electrolyte tablets, ORS solution, or broth.
Do not attempt to force rapid sodium correction in the field — the rate of sodium correction must be controlled to prevent brain injury. Give ORS steadily, not in large boluses.
If the patient has seizures, protect the airway, place on their side, and evacuate urgently. Hyponatremic seizures are not managed by giving more sodium boluses in the field — hospital electrolyte management is required.

Kit contents for dehydration management

Item	Quantity	Use
Commercial ORS sachets (e.g., Pedialyte powder, WHO ORS)	20–30 sachets	Immediate rehydration without mixing
Salt (sodium chloride), small container	Sufficient for 10 liters ORS	DIY ORS mixing
Baking soda, small container	Sufficient for 10 liters ORS	DIY ORS buffer component
"No-salt" potassium chloride substitute	Small container	DIY ORS potassium source
White sugar, sealed container	Sufficient for 10 liters ORS	DIY ORS glucose component
Oral syringes, 10 mL	5	Small-dose administration to vomiting patients and children
22 French catheter or flexible tubing (60 cm)	1–2	Proctoclysis setup
500 mL IV saline bags (0.9% NaCl)	4	IV rehydration for severe cases
IV administration set	2	IV delivery
Ondansetron (Zofran) 4 mg tablets	10	Anti-emetic for ORS tolerance
Urine color chart (laminated)	1	Ongoing hydration monitoring
Digital kitchen scale (0.1 g accuracy)	1	Precise ORS mixing

Rehydration checklist

Dehydration management does not exist in isolation. Its most common causes — diarrheal illness, gastroenteritis, heat exposure — are covered adjacent to it: hygiene and sanitation covers the prevention of the illnesses that cause most GI fluid loss; heatstroke recognition and cooling covers heat illness management where dehydration is a primary driver; and the water quality and storage pages (water foundation) ensure the ORS you mix is not itself a source of the illness you are treating. Contaminated water in ORS causes diarrhea — defeating the rehydration entirely.