Table 1 Classification of levels of evidence used in this review [18]

Level I

Strong evidence based on studies with a broad generalisability or meta-analyses based on level I studies

Level II

Moderate evidence based on prospective or retrospective studies with narrow generalisability or cohort studies, case control studies or randomised control trials

Level III

Limited evidence based on diagnostic accuracy studies with several flaws in research methods or on nonrandomised comparison studies based on outcomes

Level IV

Insufficient evidence, studies with multiple flaws in research methods or case series, descriptive studies or expert opinions