Edema is a diagnostic criterion for nephrotic syndrome, as well as a major clinical symptom and characteristic of the syndrome. There are several reasons why patients develop edema.

First, a large amount of protein leaks out from the kidneys, causing a decrease in the concentration of plasma albumin and a decrease in the colloid osmotic pressure in the plasma, which makes it easy for water to move from inside the blood vessels to outside, causing edema.

Second, patients with nephrotic syndrome have increased vascular permeability, which also makes it easy for water to enter the interstitial tissues. Among patients with nephrotic syndrome, some are prone to acute renal failure, which further decreases the kidney's ability to excrete water, and these factors together lead to the common occurrence of edema in patients.