Analysis of the stability of urea in dried blood spots collected and stored on filter paper.

The ability to use dry blood spots (DBSs) on filter paper for the analysis of urea levels could be an important diagnostic tool for areas that have limited access to laboratory facili- ties. We developed a method for the extraction and quantifi cation of urea from DBSs that were stored on 3M Whatman fi lter paper and investigated the effect of long-term storage on the level of urea in DBSs. DBSs of 4.5 mm in diameter were used for our assay, and we determined the urea levels in blood using a commercially available enzymatic kit (UV GLDH-method; Randox laboratories Ltd., UK). The DBSs on fi lter discs were stored at 4°C or at 37°C for 120 days. The mean intra- and inter-assay coeffi cient of variance for our method of urea extraction from dried blood was 4.2% and 6.3%, respectively. We collect- ed 75 fresh blood samples and compared the urea content of each fresh sample with the urea content of DBSs taken from corresponding fresh blood samples. Regression analysis reported a regression coeffi cient (r) value of 0.97 and a recovery of urea from dried spots was 102.2%. Urea concentrations in DBSs were stable for up to 120 and 90 days when stored at 4°C and 37°C, respectively. Our results show that urea can be stored and quanti- tatively recovered from small volumes of blood that was collected on fi lter paper.