Table 3 Confounders that the included studies adjusted for

Bouvy 2003 [8]

Age and gender, prior hospital admissions for congestive heart failure, diabetes and for concomitant use of diuretics, low-dose aspirin, antibiotics, paracetamol (acetaminophen), epoetin, corticosteroids, opioids, digoxin, antigout drugs and duration of use of ACE inhibitor

Huerta 2005 [34]

Sex, age, calendar year, body mass index, HF, hypertension, diabetes, antihypertensive use, oral steroid use, NSAID use, and consultant visits and hospitalizations in the previous year

Leonard 2012 [6]

Hospitalized in prior 30 days, ever past anemia, ever past coronary disease, ever past heart failure/cardiomyopathy, ever past disorders of stomach function, ever past arthropathies and related disorders, ever past pain, ever past gastrointestinal drug use, ever past cardiovascular system drug use, ever past central nervous system drug use, ever past infection-treating drug use, ever past endocrine system drug use, ever past nutrition and blood drug use, ever past musculoskeletal and joint disease drug use, frusemide use in the prior 28 days, and kidney sensitizer drug exposure in the prior 180 days

Murray 1990 [9]

Age, gender, race, coronary artery disease, baseline systolic blood pressure, diuretic use,

Perez Gutthann 1996 [36]

Age, sex, calendar year, cardiovascular risk indicator, recent hospitalization for disorders renal, exposure to NSAIDs, prescription ASA, nephrotoxic drugs

Evans 1995 [31]

Age, gender, could not find information in other covariates

Griffin 2000 [32]

Age (65–74, 75–84, > = 85), gender, ethnicity, nursing home resident, recent hospitalization (within 30 days, 31–365 days, none in the past year), concomitant use of loop diuretic, thiazide, ACE inhibitor, and antibiotics (within 30 days), prescription for allopurinol, cyclosporin, gold, sulfinpyrazone, or penidllamine, first prescription for cimetidine in the past 60 days, or procedure code Indicating intravenous radio contrast within the past 30 days

Schneider 2006 [37]

Age, gender, comorbidity (Hypertension, Diabetes, Heart failure, Cardiovascular disease, Atherosclerosis, Hyperlipidemia, Respiratory disease, Gastrointestinal ulcer disease, Chronic renal failure, Acute renal failure, Renal disease, Renovascular disease, Renal infection, Conditions secondary to renal impairment, Renal manifestation of systemic diseases, Systemic disease and malignancy relevant to renal function), drug use (Oral anticoagulants, Oral corticosteroids, Psychotropic drugs, Thyroid drugs. Current use of aspirin, Use of nephrotoxic drugs, Exposure to contrast media), comorbidity measures (No. of different drugs, Chronic disease score, Charlson index, Health care utilization (>12 physician visits, > = 1 nephrologist visits >1 hospitalization))

Lafrance 2009 [35]

Age, gender, race, concurrent disease (Arrhythmia, Chronic kidney disease, Cardiovascular disease, Cancer, Chronic liver disease, Chronic pulmonary disease, Congestive heart failure, Diabetes, Hyperlipidemia, Hypertension, Osteoarthritis, Rheumatoid arthritis, Peptic ulcer/ GERD, PVD, Valvular disease), hospitalization (last 30 days, previous year), drug use (ACEi or ARBs, Beta-blockers, Diuretics, Oral anticoagulants, Platelet aggregation drugs, Nephrotoxic drugs, Corticosteroids, Radio contrast exposure), laboratory (Serum albumin)

Henry 1997 [33]

Age, history of gout, heart disease and renal disease