Blood plasma osmolarity is the gold-standard measure of hydration. Euhydrated reads 275 to under 295 mOsm/L; 295-300 signals impending dehydration; anything above 300 means you're clinically dehydrated. You can estimate it without a lab using: Calculated Osmolarity (mmol/L) = 1.86 × (Na + K) + Glucose + Urea + 10.
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