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. 2022 Apr 13;18(8):485–498. doi: 10.1038/s41581-022-00570-3

Table 1.

Biases that can occur in epidemiological studies of kidney disease populations as a result of barriers to health care

Type of bias Barriers to accessing health care pre-pandemic Barriers to SARS-CoV-2 testing Barriers to accessing health care during the COVID-19 pandemic
Information bias (misclassification of the study exposure or outcome)

Misclassification of CKD as non-CKD (absence of testing or coding for CKD)

Misclassification of subclinical CKD as AKI and vice versa

Misclassification of infection as non-infection Misclassification of cause of hospital admission
Confounding (when a measured or unmeasured variable, not on the causal pathway, influences both the study exposure and the study outcome)

Reasons for getting a particular type of care will confound results

Applies to all kidney disease populations

Reasons for getting a test will confound results

Applies to all kidney disease populations

Reasons for getting a particular type of care will confound results

Applies to all kidney disease populations

Collider bias (when the study sample collected is conditional on a variable on the causal pathway between study exposure and study outcome) When a retrospective health record study only includes those with baseline and repeat creatinine test results (in a conventional cohort study people with missing test results would be logged as ‘lost to follow-up’ and appropriately censored/analysed) Access to SARS-CoV-2 testing may depend on COVID-19 severity and varies between kidney disease populations; comparisons of outcomes amongst those tested may be biased Access to hospital/ICU care depends on COVID-19 severity and underlying chances of survival/comorbidity, which varies between kidney disease populations; therefore, analyses of hospitalized/ICU patient populations may not always provide information on pathobiology
Selection bias (for a cohort study, selection bias is a systematic bias in the ability to capture study outcomes dependent on the (unobserved) study outcome; for a case–control study, selection bias occurs when cases are drawn from a different source population than the controls)

Not applicable for studies where COVID-19 is an outcome

For studies that consider kidney outcomes after COVID-19, barriers to kidney care/dialysis/transplantation can introduce selection bias

Especially for transplant, home dialysis, CKD and AKI populations where there was no systematic screening for COVID-19

Applies to all kidney disease populations

Some populations may have had fewer barriers than others (e.g. in-centre haemodialysis or transplant recipients)

Some populations potentially may have had less access to ICU care

AKI, acute kidney injury; CKD, chronic kidney disease; ICU, intensive care unit.