Dehydration
A deficit of total body water that impairs cellular function; caused by inadequate intake, excessive loss, or both.
The NCLEX rarely names “dehydration” outright — it hands you a vignette (NPO post-op, vomiting, a febrile child, diuretic use) and asks the priority assessment or intervention. The highest-yield tell is the lab combo: elevated BUN with a normal creatinine (BUN:creatinine ratio >20:1), rising hematocrit, and hypernatremia (sodium >145 mEq/L) from hemoconcentration. When stems offer competing actions, isotonic IV fluid (0.9% normal saline) to restore intravascular volume beats treating the tachycardia or giving an antiemetic, because perfusion is the priority.
Don’t confuse fluid-volume deficit with edema, which signals fluid overload — and remember daily weight is the most accurate fluid-status measure (a 1 kg change ≈ 1 L), more reliable than intake/output. The classic trap: a falling BP feels alarming, but hypotension is a LATE sign — tachycardia, restlessness, and flat neck veins come first, and an untreated deficit progresses to hypovolemic shock. Hook: dehydrated blood is thick, so the numbers go up (HCT, BUN, sodium) while urine output and skin turgor go down.
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