This study investigates the effect of background knowledge in languages for specific academic purposes (LSAP) tests. Following the observation of previous studies that the effect of background knowledge varies according to the level of language proficiency, the study applied the theory of linguistic thresholds to LSAP reading tests. The research question was whether students would be able to draw on their background knowledge only at certain levels of second language (L2) proficiency. International students in Germany took part in a study based on two LSAP reading tests from different subject areas. The texts were subject specific but did not assume previous knowledge. Language proficiency was assessed by C-tests; background knowledge by three different variables. The effect of background knowledge on reading test performance was strong. Whilst the effect of background knowledge did vary somewhat according to the level of language proficiency, the findings were inconclusive in relation to possible thresholds. In the case of the employed LSAP tests, which were not highly specific, they were fuzzy or non-existent and the concept seems inappropriate in relation to language tests for academic purposes.