This prospective double-blind randomized crossover study used the Ocular Surface Disease Index ( OSDI ) , tear film break-up time ( TBUT ) , Schirmer 's Test , and OXFORD Scale at baseline and after each of two 1-month treatment periods to measure the effect of 20 % diluted AS eye drops vs. PFAT in 20 consecutive severe DES patients that were refractory to conventional treatment .