Would you perform a kidney biopsy to rule out other etiologies before diagnosing Loin Pain Hematuria Syndrome in a patient with persistent microscopic hematuria, left flank pain, no proteinuria, normal renal function, normal cystoscopy, and normal imaging?

Probably not unless something changes.

Probably not, because even if they turn out to have something like IgA nephropathy or thin basement membrane disease, I would not do anything different. Genetic testing, since it is noninvasive, may be more reasonable.