What is considered the gold standard test for diagnosing nephrotic syndrome?

The gold standard test for diagnosing nephrotic syndrome is a 24-hour urine collection. This test is critical because nephrotic syndrome is primarily characterized by a significant loss of protein in the urine, specifically more than 3.5 grams of protein in a 24-hour period.

The 24-hour urine collection allows for accurate quantification of proteinuria, confirming the diagnosis. While serum creatinine can provide information about kidney function and renal ultrasound can help assess the kidney's structure, neither directly diagnose nephrotic syndrome nor measure protein loss comprehensively. A kidney biopsy, although a definitive diagnostic tool in many renal disorders, is not routinely required to diagnose nephrotic syndrome when the clinical presentation and laboratory findings are sufficient. Instead, it is typically reserved for cases where there is uncertainty in the diagnosis or when a secondary cause for the nephrotic syndrome needs to be identified.