Study Design and Patients
This was a retrospective cohort study, included patients admitted to the Nephrology ward of Hospital São Paulo, Universidade Federal de São Paulo, São Paulo, Brazil. Patients were eligible if submitted to percutaneous kidney biopsy performed with aged ≥16 years as indicated by the nephrologists. Exclusion criteria were kidney transplant recipients. In our center, all percutaneous native kidney biopsies were performed in patients admitted exclusively to the nephrology ward. We did not perform native kidney biopsy in outpatient setting. Data were obtained, between January 2008 and December 2014, from patients’ medical records and the electronic hospital database. The follow-up period underwent 12-month after the kidney biopsy.
Demographic and clinical data
We analyzed the following variables: age, sex, race, body mass index (BMI), major comorbidities such as hypertension, diabetes mellitus (DM), cancer, cardiovascular disease (CVD including heart failure, coronary artery disease, stroke, and peripheral artery disease), and systemic lupus erythematosus (SLE).
We evaluated the use of diuretics, antibiotics, statins, angiotensin-converting enzyme inhibitors (ACEI), and/or angiotensin receptor blockers (ARB), nonsteroidal anti-inflammatory drugs (NSAID), and contrast media (recent use as well as administration during hospitalization). Previous use of corticosteroids was also recorded.
Indications for renal biopsy were nephrotic syndrome (characterized by edema, hypoalbuminemia [<3.5 g/L], dyslipidemia, and proteinuria ≥3.5 g/24 hours [h]), or nephrotic-range proteinuria (proteinuria ≥3.5 g/24 h), rapidly progressive glomerulonephritis (RPGN, characterized by hematuria and/or proteinuria on urinalysis associated with rapid decline in renal function indicated by progressively increasing serum creatinine levels, which may occur over days, weeks, or months prior to renal biopsy), sub-nephrotic proteinuria (isolated proteinuria >1.0 g/24 h), dysmorphic hematuria associated with some grade of proteinuria, renal dysfunction if unknown origin, and renal manifestations of systemic diseases.
Evaluation performed upon admission
The following parameters were evaluated upon admission: hemoglobin, serum sodium, potassium, ionic calcium, phosphorus, and albumin levels, pH and bicarbonate levels, serum total cholesterol and its fractions, serum triglycerides, serum creatinine and urea levels, as well as 24-h urinary protein excretion. The glomerular filtration rate (GFR) was estimated using the Modification of Diet in Renal Disease Study Group (MDRD) equation (6).
All patients underwent kidney and urinary tract ultrasonography. The length of the kidneys was recorded in all patients.
Optical microscopy analysis and immunofluorescence studies were performed for histopathological diagnostic confirmation as well electron microscopy, when indicated. All analysis was performed by a single renal pathologist.
We evaluated the serum creatinine level upon admission, during hospitalization, and specific time-points after discharge until 12 months after the kidney biopsy. The reading that showed the lowest level during hospitalization was considered the baseline serum creatinine level because laboratory data prior to hospitalization were unavailable. The highest serum creatinine level during hospitalization was used to diagnose AKI.
Acute kidney injury
We defined and classified AKI observed during hospitalization based on the Kidney Disease: Improving Global Outcomes (KDIGO) criteria using only serum creatinine levels (7). AKI was defined as an increase in serum creatinine ≥0.3 mg/dL over ≤48 hours or a ≥1.5-fold increase in serum creatinine over baseline levels within the 7 days. AKI was classified into 3 stages as follows: stage 1: ≥1.5 to 1.9-fold increase in serum creatinine levels or an increase of 0.3 mg/dL compared with baseline levels, stage 2: ≥2.0 to 2.9-fold increase in serum creatinine levels compared with baseline levels, and stage 3: >3.0-fold increase in serum creatinine levels compared with baseline creatinine levels or serum creatinine levels >4.0 mg/dL with an increase of at least 0.5 mg/dL, or the initiation of renal replacement therapy.
The following outcomes were assessed: kidney function (evaluated by serum creatinine levels), need for dialysis initiation, and mortality within 12 months after kidney biopsy.
Data were expressed as mean and standard deviation for normally distributed variables and as median and quartiles (25–75%) for variables showing non-normal distribution. Parametric distribution was confirmed using the Shapiro-Wilk test. We used the Student's t-test or the Mann-Whitney test for numerical variables and the Chi-square test for comparison between nominal variables.
For covariance analyze repeated measures of creatinine and for intergroup comparison of data, we used the Generalized Estimation Equations and applied the autoregressive structure correlation matrix (1) and gamma distribution with log link.
Variables that were significantly associated with AKI on univariate analysis (p<0.05) were subjected to multivariate logistic regression analysis for variables such as age and sex.
A p value <0.05 was considered statistically significant. SPSS software, version 20.0 (IBM Corp, USA, 2011) was used for statistical analysis.