Abstract
Therapeutic drug monitoring for biologic agents such as infliximab and adalimumab is complicated when anti-drug antibodies are
present, since several widely used drug-level assay formats are themselves affected by antibody interference in ways that can
produce a falsely low or falsely normal reported drug level. We report a short series of nine patients from a single rheumatology
biologics clinic in whom drug level results were discordant with clinical response, prompting repeat testing by an alternative
assay format; in seven of the nine, the discordance resolved once antibody interference with the original assay format was
identified and accounted for. We summarise the interference mechanisms relevant to the two most common assay formats and
recommend a practical trigger for when an unexpected drug level result should prompt assay-format cross-checking before a
clinical dose decision is made on it.
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