a gross filter in a drainage device for filtering fluids drained from the pleural cavity of a patient comprising a filter body having a first side panel , second side panel , back panel and bottom panel that assemble to form a generally box - shape filter with open front and top portions . the gross filter is located in the first of two subchambers that comprise the collection chamber . the filter body further comprises a filter screen sized for filtering gross particulates , such as blood clots , that may become entrained in fluid drained from a patient . the gross body is configured so as to permit the further filtering of fluids when blood clots have formed and blocked the filter screen of the filter &# 39 ; s bottom panel from fluid flow therethrough , thereby allowing uninterrupted filtering of blood into the collection chamber through the first and second side panels instead . to further facilitate the filtering of blood through the filter body when a blocked state exists at the bottom panel portion thereof , the first and second side panels are angled toward each other at their bottom ends and gradually spaced apart from the collection chamber walls in order to permit unimpeded filtering of fluid through the side panels . the open front portion of the gross filter permits clear viewing by medical personnel of the amount of blood clots being retained inside the filter body so that measuring indicia may be provided on the drainage unit &# 39 ; s front panel as an indication of the amount and rate of blood clot retention inside the filter . an overflow filter portion is also provided at the top portion of one of the side panels for filtering fluid when the rest of the filter body is clogged and blocked by blood clots or when the first subchamber is filled to capacity .