What is the anion gap range for determining the severity of dehydration?

Anion Gap Is Not Used to Assess Dehydration Severity

The anion gap is not a marker for dehydration severity and should not be used for this purpose. Dehydration assessment relies on serum osmolality, not anion gap, which is a tool for evaluating metabolic acidosis and specific toxicological conditions.

The Correct Marker: Serum Osmolality for Dehydration

Directly measured serum or plasma osmolality >300 mOsm/kg defines dehydration in clinical practice 1. This is the gold standard recommended by the ESPEN guidelines and endorsed by the US Panel on Dietary Reference Intakes for Electrolytes and Water 1.

Osmolality Thresholds:

• Normal hydration: <300 mOsm/kg 1
• Dehydration: >300 mOsm/kg 1
• Increased mortality risk: Serum osmolality >300 mOsm/kg is associated with doubled risk of 4-year disability and increased mortality 1

When Direct Osmolality Is Unavailable:

• Use the calculated osmolarity equation: osmolarity = 1.86 (Na+ + K+) + 1.15 glucose + urea + 14 (all in mmol/L) 1
• Action threshold: >295 mmol/L suggests dehydration 1

What Anion Gap Actually Measures

The anion gap reflects unmeasured anions in metabolic acidosis, not hydration status. It is calculated as: [Na+ + K+] - [Cl- + HCO3-] 1.

Normal Anion Gap Ranges (Modern Ion-Selective Electrode Methods):

• Normal range: 3-11 mmol/L or 5-12 mmol/L 2, 3
• Low anion gap: <3 mmol/L 2, 3
• Elevated anion gap: >12 mmol/L 2

Clinical Contexts Where Anion Gap Matters:

Diabetic Ketoacidosis (DKA):

• Mild DKA: anion gap >10 mEq/L 4
• Moderate to severe DKA: anion gap >12 mEq/L 4
• Anion gap of 22 mmol/L strongly suggests DKA 5
• Resolution requires anion gap normalization to ≤12 mEq/L 6

Ethylene Glycol Poisoning:

• Strong indication for extracorporeal treatment: anion gap >27 mmol/L 1
• Weak indication: anion gap 23-27 mmol/L 1

Severe Metabolic Acidosis:

• Anion gap >24 mmol/L suggests significant metabolic acidosis requiring urgent evaluation 2
• Values >30 mmol/L are uncommon and may indicate severe acidosis or laboratory error 7

Critical Pitfall to Avoid

Do not confuse dehydration with metabolic acidosis. Dehydration (low-intake dehydration) causes raised serum osmolality with normal or minimally elevated individual electrolytes 1. The anion gap remains normal in pure dehydration unless there is concurrent metabolic acidosis from another cause 1.

Why This Matters:

• In low-intake dehydration, despite raised osmolality, sodium, potassium, urea, and glucose often remain within normal ranges—they simply rise together proportionally 1
• Extracellular water loss from vomiting or diarrhea (volume depletion) is associated with normal or low plasma osmolality, not elevated 1
• Clinical signs like skin turgor, mouth dryness, and urine color should NOT be used to assess hydration status in older adults 1

Practical Algorithm for Dehydration Assessment

1. Measure serum osmolality directly (preferred method) 1
2. If >300 mOsm/kg: confirm glucose and urea are normal to exclude other causes of hyperosmolality 1
3. If direct osmolality unavailable: calculate osmolarity using the formula above 1
4. If >295 mmol/L calculated: treat as dehydration 1
5. Never use anion gap for dehydration assessment—reserve it for evaluating metabolic acidosis 1, 2