Background: Modified Light's criteria are widely used to categorize pleural fluids as either exudates or transudates. Similarly, the serum-ascites albumin gradient (SAAG) is used in the differential diagnosis of ascites, particularly with reference to the prediction of portal hypertension. Fluid and serum samples are required for both of these to be applied. The effect of the time interval between fluid and serum samples on the interpretation of results has not been studied. Methods: We examined the effect of sample timing on (a) the application of modified Light's criteria, and (b) the categorization of SAAG as wide (≥11 g/L) or narrow (<11 g/L). Specifically, we compared the use of a 'routine' serum sample, i.e. one that was not formally paired by the requesting clinician with the fluid sample, with serum samples collected within 2 h of the fluid sample. Results: Of 77 pleural fluids included for analysis, 45/47 were categorized as exudates, and 32/30 as transudates, using near-simultaneous/routine serum samples respectively. Discrepant categorization was observed in two cases (P=0.74). Of 109 ascitic fluids, SAAG was ≥11 g/L in 100/95 cases, and <11 g/L in 9/14, using near-simultaneous/routine serum samples respectively. Discrepant categorization was observed in five cases (P=0.27). Conclusions: With reference to categorizing pleural and ascitic fluids as described above under (a) and (b), in most cases the use of routine serum samples does not alter the fluid categorization compared with the use of serum samples collected within 2 h.