Table 1

Eligibility criteria

Domain	Inclusion	Exclusion
Population	KQ A–E: publications explicitly referring to multiple chemical sensitivity (MCS) as well as publications that are consistent with the working definition of MCS; in these cases, publications may refer to chemical intolerance, idiopathic environmental intolerance, chemical sensitivity or hypersensitivity, ICD-10-CM code F45.9 (somatoform disorder, unspecified) or symptoms associated with environmental factors. KQ F: treatment or management studies that include participants of all ages that explicitly refer to MCS or whose description aligns with the working definition of MCS.	KQ A–F: publications primarily addressing other conditions such as sick building syndrome, electromagnetic sensitivity, Gulf War syndrome or chemical sensitivity not consistent with the working definition.
Intervention/ Independent variable	KQ A: publications that describe a definition or diagnostic criteria for MCS together with a validation process providing content, construct or criterion validity. Approaches may include a consensus development process for content experts, documentation of empirical concurrent and discriminant validity or other approaches. KQ B: studies that evaluate the performance of a diagnostic test, diagnostic criteria or other diagnostic tool to identify people with MCS. KQ C: studies that report prevalence or incidence of MCS in a general population sample. KQ D: studies that address whether MCS is a distinct condition and that provide empirical evidence of discriminatory power or that are based on formal expert consensus methods. KQ E: studies that evaluate a biological mechanism for the development or progression of MCS (either using the term MCS or describing a condition consistent with the working definition of MCS). KQ F: studies evaluating interventions aimed at managing or treating MCS. Interventions will not be restricted by content or treatment approach and may include interventions aimed at coping with MCS symptoms, as well as those addressing the underlying causes of MCS.	KQ B: studies only describing the psychometric properties without data on diagnostic performance. Studies reporting on MCS in preselected samples, such as patients in an environmental health clinic. KQ D: publications not including original data and opinions of individual authors without formal consensus regarding whether or not MCS is a distinct disorder. KQ E: studies that only report on exposures or risk factors without addressing the underlying biological or physiological mechanism, and studies addressing potential psychosocial mechanisms of action (eg, attribution errors). KQF: interventions not aimed at managing or treating MCS.
Comparator	KQ A, D, E: any or no comparator. KQ B: studies with a reference standard. The reference standard may be a clinical interview with a healthcare professional or other method of determining all defining criteria of MCS are met according to a published definition of MCS. KQ C: studies reporting a numerator and denominator. KQ F: evaluations with historic (pre-post, time series) or concurrent (randomised controlled trial, controlled clinical trial, cohort studies comparing two observational cohorts).	KQ F: uncontrolled case studies and case series without numerical baseline assessments.
Outcome	KQ A: MCS definition or operationable diagnostic criteria. KQ B: diagnostic accuracy outcomes (eg, sensitivity, specificity, accuracy, area under the curve, receiver operating characteristics, positive predictive value, negative predictive value, false and true positives and negatives, congruence with diagnosis). KQ C: sufficient detail provided to determine the denominator and numerator for prevalence or incidence. KQ D: original empirical data that provide direct evidence, mechanistic evidence and parallel evidence aiming to establish MCS as a disorder. KQ E: empirical strength of association, consistency, reversibility, specificity, temporality, biological gradient, plausibility, coherence, experimental or analogue evidence. KQ F: patient health (self or clinician report), physiological measures assessing the effect of the intervention (effectiveness as well as safety indicators), or other evaluations of the impact of the intervention; quantitative and qualitative analyses are eligible.	KQ D: secondary literature only citing other studies and publications not explicitly referring to MCS. KQ F: studies reporting only on treatment uptake, patient or provider acceptability of treatments or treatment costs.
Other limiters	English-language peer-reviewed literature and research trial records	Studies published in abbreviated form (eg, conference abstracts).

KQ A: Which definitions of MCS have been validated? KQ B: What is the diagnostic performance of tools for identifying MCS? KQ C: What is the prevalence and incidence of MCS? KQ D: What is the empirical evidence that MCS is a distinct disorder? KQ E: What is the empirical evidence for underlying biological mechanisms for MCS? KQ F: What are the effectiveness and safety of treatment and management strategies for MCS?
ICD-10-CM, International Classification of Diseases, Tenth Revision, Clinical Modification; KQ, key review questions; MCS, multiple chemical sensitivity; N/A, not available.