The most important message is that FSGS is a lesion and not a disease. The finding of an FSGS lesion is the start of an exploratory process leading to a diagnosis and not an end in itself. We propose that adult patients presenting with nephrotic syndrome, a FSGS lesion on LM, extensive foot process effacement (>80%) on EM examination and no factors associated with causing FSGS are likely to have primary/idiopathic FSGS.

Conversely, the presence of subnephrotic or nephrotic-range proteinuria, but no nephrotic syndrome, in a patient with segmental foot process effacement on EM suggests we are dealing with secondary FSGS, although the cause may not be obvious with the available diagnostic tools.

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It should be understood that morphology alone (e.g. without any clinical information) makes it very difficult or impossible to reliably distinguish a primary from a secondary form of FSGS. Thus primary and secondary FSGS might be viewed as clinician-pathological entities and is important for clinicians to understand they have to integrate the pathological findings with clinical information to arrive at the most precise separation of primary from secondary FSGS.

Differentiating primary from secondary FSGS is crucial. Patients with primary FSGS should be considered for immunosuppressive treatment, whereas patients with secondary FSGS should be treated conservatively with therapy aimed at reducing BP and proteinuria.

Long-term follow-up is needed because clinical presentation and renal biopsy findings may suggest a primary FSGS going into spontaneous remission or a secondary FSGS progressing to full nephrotic syndrome. In this last group, a repeat renal biopsy should be considered.

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