This exact question is in asked in the NBME. They say the next best step is to check for orthostatic proteinuria. I'll copy the question explanation below.
The first approach in assessing incidentally found proteinuria in a healthy asymptomatic child is to repeat the urinalysis. The most common cause of proteinuria in asymptomatic children is transient proteinuria secondary to other causes such as fever, stress, exercise, and dehydration. In addition, proteinuria can be orthostatic or postural, with increased protein in a standing or upright position that normalizes when a patient is supine or recumbent. Orthostatic proteinuria is common in adolescent males. A repeat urinalysis on awakening would likely show a normal urinalysis, and a urinalysis at bedtime would show proteinuria. In a patient with persistent proteinuria, further workup should be done to evaluate kidney disease.
Source: Other - NBME Self assessment pediatrics form 8 Q46
- Most common cause of isolated proteinuria in children
