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Box 3 Renal biopsy during vasculitides

From: French recommendations for the management of systemic necrotizing vasculitides (polyarteritis nodosa and ANCA-associated vasculitides)

When diagnosing systemic vasculitis, a renal puncture biopsy with optical microscopy and immunofluorescence examination is:  
• Recommended if there is proteinuria compatible with a glomerular origin (made up of > 60% albumin) and microscopic hematuria  
• Recommended if there is a deterioration in kidney function (increase in serum creatinine and/or decrease in estimated glomerular filtration rate, eGFR), in the absence of an identifiable cause  
• Discussed in the case of isolated hematuria, after ruling out a pathology of the urinary tract, or in the case of isolated proteinuria, in the absence of another identifiable cause  
Conversely, it is contraindicated in cases of suspected PAN (where the impairment is most often vascular and not glomerular), due to often severe high blood pressure and the risk of bleeding related to the frequent existence of renal microaneurysms, which must be detected by imaging  
The diagnostic value of the renal needle biopsy is most important in the first month after starting the treatment  
In the immediate aftermath of vasculitis with histologically confirmed renal impairment, a control biopsy may be proposed in the event of worsening of the kidney function under treatment not explained by the initial histology (sampling problem or refractory form)  
A while after the initial flare-up, it will be indicated in the event of renal signs which can suggest a relapse of renal vasculitis: reappearance of microscopic hematuria, rapid deterioration of kidney function (serum creatinine, eGFR) with an increase in proteinuria, in the absence of another identifiable cause and other documentation of the relapse  
On the other hand, the mere persistence of low abundance microscopic hematuria or proteinuria following a flare-up of renal vasculitis does not justify a repetition of the kidney biopsy  
The kidney biopsy confirms the diagnosis of vasculitis combined with ANCA, by showing necrotizing glomerulonephritis without deposits of immunoglobulins or a supplement (called pauci-immune extracapillary glomerulonephritis), and sometimes impairment to intrarenal arterioles, or even rarely granulomatous lesions in GPA. It makes it possible to rule out differential diagnoses in cases of glomerulopathy with the presence of ANCA (infectious endocarditis, vasculitis with anti-glomerular basement membrane antibodies, lupus glomerulonephritis, glomerulonephritis combined with inflammatory bowel disease, etc.) or to show the presence of renal lesions secondary to another pathology (diabetes, high blood pressure), unrelated to vasculitis. It is essential in the rare cases of systemic necrotizing vasculitis with a negative immunological assessment and, in particular, negativity of ANCA or in case of the presence of both ANCA and anti-glomerular basement membrane antibodies (anti-MBG)  
In addition, the kidney biopsy makes it possible to specify the renal prognosis, in particular, due to the classifications proposed by Berden et al. (distinguishing focal, crescent, mixed, or sclerotic forms) and Brix et al. (quantifying interstitial fibrosis and tubular atrophy and the percentage of normal glomeruli)  
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