The full scoring system along with commentary can be found in this paper published in a 2008 issue of the American Journal of Transplantation. As I wanted to review this prior to taking my ASN in-service exam tomorrow, I'll write out the salient details of the Classification system.
Class 1 is a "normal biopsy."
Class 2 is "antibody-mediated changes." Ideally, both positive C4d staining and circulating donor-specific antibodies are present in the setting of a rising creatinine to make this diagnosis. In acute antibody-mediated rejection, there are three variants: (i) an ATN-like picture, (ii) capillary involvement, or (iii) arterial involvement. In chronic antibody-mediated rejection, there is evidence of chronic tissue injury such as glomerular double contours, peritubular capillary basement membrane multilayering, interstitial fibrosis/tubular atrophy (IFTA), or fibrous intimal thickening in arteries.
Class 3 refers to "Borderline Changes" which is essentially a mild form of T-cell-mediated rejection. This category is used when there is no intimal arteritis present, but there are foci of tubulitis or minor interstitial infiltration.
Class 4 is a more full-blown form of T-cell mediated rejection. As with humoral rejection, there are both acute & chronic forms:
The acute form of T-cell mediated rejection is furthermore subclassified as follows. Since this is the most common form of rejection, it is useful to know:
Class IA: there is at least 25% of parenchymal showing interestitial infiltration and foci of moderate tubulitis (defined as a certain number of immune cells present in tubular cross-sections).
Class IB: just like Class IA except there is more severe tubulitis.
Class IIA: there is mild-to-moderate intimal arteritis.
Class IIB: there is severe intimal arteritis comprising at least 25% of the lumenal area.
Class III: there is transmural (e.g. the full vessel wall thickness) arteritis.
Class 5 refers to interstitial fibrosis and tubular atrophy (IFTA), which is the new preferred term for "chronic allograft nephropathy." Grade I refers to <25%>50% of cortical area involved.
Class 6 is a catch-all term describing changes not considered to be due to rejection--for example, recurrent FSGS or CNI toxicity.