the next video in my clinical scale series is going to be on lights criteria in my opinion lights criteria is actually the easiest clinical scale that you need to know for USMLE comlex shelf exams and rotations but plural affusions are conceptually a lot more difficult to understand than the topics that the other clinical scales test on and so for that reason my goal in this video is not just to teach you light's criteria but to also help your brain make sense of the difference between an exudative and transid dative plural Fusion so in short a the lights criteria is used to identify an exudative plural affusion when we look at plural affusions we categorize them as either exudative or transid dative and I'm skipping ahead a little bit but if any of the light's criteria are true you have an exudative plural Fusion for the sake of Simplicity I want you to think of exudative as extra stuff right exudative equals extra if a plural affusion is exudative there's extra stuff in it so exudative plural affusions are caused by states of increased capillary permeability some examples of disease states that cause increased capillary permeability include any type of inflammation pneumonia cancer TB or pulmonary emilii so exida equals equals extra stuff in the plural Fusion on the other hand transid dative plural fusions are caused by states of fluid overload due to either increased capillary pressure or decreased oncotic pressure and some disease states that cause transid dative plural affusions are heart failure sorosis and nephrotic syndrome so for the sake of Simplicity and I'm going to repeat myself because it's very important exida equals extra stuff transid equals fluid overload if you can keep that simple in your brain then you'll be able to answer most of your test questions on plural affusions but now let's get into the actual criteria so when it comes to lights criteria there are three criteria the first is that the ratio of plural protein to serum protein is greater than 0.5 the second is that the ratio of plural LDH to serum LDH is greater than 0.6 and third the plural LDH is greater than 2/3 the upper limit of normal of specifically the serum LDH now if this seems overwhelming and random and lots of things and ratios and numbers I'm I want to simplify this so again light's criteria if any one of these three are positive you have an exudative plural affusion which is to say that if you have any one of these three criteria positive there's extra stuff due to increased capillary permeability so under what circumstances would you expect to see more stuff in a plural Fusion well if we look at these criteria if the plural protein is more than the serum protein right if the numerator is higher than the denominator that means there's more protein in the plora than protein in the serum and how do you get more protein in the plora well you get more protein in the plora when you have inflammation and cells breaking so in any inflammatory process in any cancerous process you have cells breaking open protein being released that causes more protein in the plora than there is protein in the serum and that's why for lights criteria number one that ratio would be greater than 0.5 because again the numerator would be higher than the denominator there'd be more stuff in this case protein in the plur than in the serum the same is true of the second and third criteria so if we look at those criteria they would be positive in States where LDH is higher in the plora than in the serum LDH is an enzyme that leaks out of cells when cells break and under what conditions do cells break well cells break open in states of inflammation in states of cancer tuberculosis pulmonary emilii so when cells are breaking open and releasing LDH we have an increase of LDH in the plora more than an increase in LDH in the serum and when that happens when the numerator in this case plora is higher than the denominator in this case serum that ratio will be greater than 0.6 or greater than 2/3 the upper limit of normal of the serum LDH so again in an effort to help you simplify this and understand conceptually the difference between exudate and transudate LDH leaks and protein permeates normally LDH is tightly compacted into cells and protein does not freely permeate across barriers but when cells break and you have exudative plural effusions or disease processes that cause exudative plural effusions LDH leaks protein permeates so again I already alluded to this but if any one of these three lights criteria are positive you have an exudative plural affusion so on usmon complex if you have high plural protein High plural LDH chances are you're looking at an exudative plural affusion the specific criteria are shown here and these are much more useful on step two level two shelves and rotations but this can show up on step one and certainly the difference between exudative and transudative shows up all the time