A renal biopsy yields a small piece of renal parenchyma for histopathologic examination. Because many renal diseases have essentially indistinguishable clinical ﬁndings, renal biopsy is often crucial for establishing the correct diagnosis and devising an effective treatment plan. The procedure is generally uncomplicated and, in most cases, can safely be performed by a nephrologist at the bedside.
RENAL BIOPSY: INDICATIONS AND STRUCTURE OF TYPICAL SPRING-LOADED NEEDLE
The major indications for renal biopsy include renal failure of unknown cause, proteinuria, hematuria, and renal transplantation.
Proteinuria. In patients with mild proteinuria (1 to 2 g/day) that has no obvious cause, such as diabetes mellitus, a renal biopsy may be performed to establish a deﬁnitive diagnosis. The exact threshold for biopsy differs across practitioners and depends on individual clinical judgment. Possible causes of this degree of proteinuria include glomerulonephritis and mild forms of the diseases that typically cause nephrotic syndrome, such as focal segmental glomerulosclerosis (FSGS) or membranous nephropathy (MN). Although tubulointerstitial disease commonly causes mild proteinuria, a biopsy is generally not required to establish the diagnosis.
In patients with nephrotic-range proteinuria (i.e.,3 g/day), a renal biopsy is indicated to identify the disease process, guide treatment, and determine prognosis. Possible causes of nephrotic-range proteinuria include primary or secondary FSGS, MN, minimal change disease (MCD), and (rarely) ﬁbrillary or immunotactoid glomerulonephritis. If, however, the patient has a diagnosed systemic illness that is known to cause nephrotic syndrome, a renal biopsy is typically not required. Examples include patients with long-standing diabetes mellitus and concurrent diabetic retinopathy, or patients with amyloidosis seen on a biopsy of another affected organ system. In addition, young children with nephrotic syndrome are generally presumed to have MCD, with a renal biopsy only performed if empiric treatment for this condition fails.
Hematuria. In patients with gross or microscopic hematuria, the initial workup should focus on urologic abnormalities, such as nephrolithiasis, neoplasm, or infection. The presence of dysmorphic red cells, proteinuria, and renal insufﬁciency, however, strongly points toward glomerular disease. Many renal diseases are associated with microscopic hematuria, including essential hematuria, acute interstitial nephritis, IgA nephropathy, membranoproliferative glomerulonephritis, postinfectious glomerulonephritis, lupus nephritis, cryoglobulinemia, ﬁbrillary/immunotactoid glomerulonephritis, ANCA-associated vasculitis, malignant hypertension, atheroembolic renal disease, renal infarction, thrombotic microangiopathy, Henoch-Schönlein purpura, thin basement membrane nephropathy, hereditary nephritis, and anti-GBM disease. A kidney biopsy is essential for establishing the correct diagnosis and determining an optimal treatment plan.
Occasionally, patients may have isolated hematuria (i.e., without proteinuria or renal insufﬁciency). The differential diagnosis for such patients includes thin basement membrane disease, mild IgA nephropathy, and hereditary nephritis. A kidney biopsy is typically not performed, however, because treatment is not instituted unless there is signiﬁcant proteinuria or renal insufﬁciency.
Renal transplant. Patients who have undergone renal transplant and subsequently develop renal failure should have a biopsy if their renal function does not improve after provision of intravenous ﬂuids. In such circumstances, a biopsy is helpful for differentiating between various entities, such as acute or chronic
rejection, drug toxicity (especially from calcineurin inhibitors), and BK virus infection. Some centers also routinely take biopsies from transplanted kidneys at predetermined time points, even in the absence of overt dysfunction because some renal disease may initially be clinically silent.
Before a patient undergoes a renal biopsy, anticoagulation medications should be stopped, and bleeding risk should be evaluated by obtaining a prothrombin time, partial thromboplastin time, and platelet count. Any bleeding diathesis should be corrected, if possible, before the procedure.
Most patients can undergo a percutaneous biopsy, which is performed at the bedside; however, select patients may require alternate approaches, including open, laparoscopic, and transjugular biopsies. The major indications for these techniques include an uncorrectable bleeding diathesis, morbid obesity, solitary kidney, infection of the skin over the kidneys, and failed percutaneous attempts.
For a percutaneous biopsy, most patients should be placed prone, with a folded pillow under the abdomen. An ultrasound is performed to visualize the kidney and determine the location and angle of needle insertion. The upper or lower pole should be targeted so that only cortical tissue is acquired. Hydronephrosis, multiple cysts, or small hyperechoic kidneys may be seen, which increase the bleeding risk and should be considered relative contraindications.
Once the initial ultrasound is complete, the site is dressed and draped in normal sterile fashion. The site is injected with a local anesthetic, and a scalpel may be used to nick the skin at the area of planned needle insertion. The biopsy needle, which consists of a spring- loaded outer cannula and inner stylet, is then cocked as shown in the diagram. The needle is passed through the skin into the renal parenchyma, often using ultrasound for real-time guidance. The patient is instructed to hold his or her breath, and then the actuator button is depressed, causing rapid advancement of both the inner stylet and outer cannula to the device’s predetermined penetration depth. A tissue core is acquired as the cannula rapidly passes over the stylet. Two or three cores should be acquired to ensure an adequate sample. The adequacy of the tissue cores can be assessed using low-power microscopic examination. An adequate sample should contain a minimum of 8 to 10 glomeruli. The cores should be transported in normal saline to the pathology laboratory or placed in ﬁxatives if the laboratory is not on site. A renal pathologist then examines the tissue using light microscopy, electron microscopy, and immunoﬂuorescence or immunohistochemistry. Typical routine stains for light microscopy include hematoxylin and eosin, periodic acid–Schiff, Jones’ silver methenamine, and trichrome.
The main complications of a renal biopsy include bleeding, pain, damage/puncture of surrounding structures (liver, spleen, bowel), and arteriovenous ﬁstula formation. Bleeding is by far the most common complication, and it can occur into the urine collecting system, perinephric space, or subcapsular space. Patients should thus be monitored for approximately 4 to 6 hours after the procedure, with vital signs, hemoglobin levels, and urine color noted. Some centers perform a follow-up computed tomography (CT) scan or ultra-sound a few hours after the biopsy. In the event of a major bleed, transfusions or therapeutic procedures (e.g., angioembolization or laparotomy) should be performed as needed. In very rare cases, a renal biopsy results in kidney loss or death.