Criteria
The diagnosis of IgAN requires a kidney biopsy. No accumulation of clinical and laboratory evidence has sufficient specificity and sensitivity to avoid the need for diagnostic biopsy. Mesangial IgA deposits are the defining hallmark of the disease and can be identified by immunofluorescence or immunoperoxidase techniques. Mesangial deposition of IgA, along with other immunoglobulin classes and complement may also be a feature of lupus nephritis and therefore systemic lupus erythematosus should always be considered when making a diagnosis of IgAN.
While not diagnostic criteria per se, a number of histopathologic classifications have been published for IgAN. The most widely validated of these is the Oxford classification.[2]
The original Oxford classification, published in 2009, comprises four independent morphologic lesions that have high inter- and intra-observer reproducibility and are associated with a poor prognosis independent of clinical features at the time of kidney biopsy:
Mesangial hypercellularity (M0 or M1)
Endocapillary hypercellularity (E0 or E1)
Segmental glomerulosclerosis (S0 or S1)
Tubular atrophy/interstitial fibrosis (T0, T1, or T2)
Since its publication, the Oxford classification has been validated in multiple patient cohorts from North America, Europe, and Asia.[3][4][5]
In the updated Oxford Classification, published in 2017, crescents were added to the four original lesions: C0 (no crescents), C1 (crescents in less than 25% of glomeruli), and C2 (crescents in 25% or more of glomeruli), forming a new MEST-C score.[6] This is the result of a working subgroup of the IgAN Classification Working Group that demonstrated crescents (C1 or C2) were independent predictors of kidney outcomes in a pooled cohort of 3096 patients.[7] The other notable addition to the updated Oxford Classification 2017 is the subclassification of segmental sclerosis, identifying those cases with evidence of podocyte hypertrophy and/or tip lesion.
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