Why might a postoperative patient have a 24-hour intake greater than total output during the first 2 to 5 days after surgery?

• A. The stress response causes fluid retention for the first 2 to 5 days postoperatively.
• B. The patient is drinking excessively.
• C. The kidneys are producing more urine.
• D. There is measurement error.

Answer: (A)

Postoperative fluid balance is influenced by the body's stress response to surgery. Tissue injury triggers a surge of hormones—such as antidiuretic hormone (ADH), aldosterone, and cortisol—and inflammatory mediators that help preserve perfusion to healing tissues. These changes cause the kidneys to conserve water and sodium and can lead to fluid shifting into the interstitial space (third-spacing). Combined with ongoing IV fluid administration after surgery, this hormonal response often produces a positive fluid balance where intake exceeds output for the first few days, typically the first 2 to 5 days. While drinking heavily or measurement quirks could play a role in some cases, the usual explanation is fluid retention driven by the postoperative stress response.