Free secretory component as a standardization protein for nasopharyngeal specimens from children with upper respiratory tract infection

Free secretory component (FSC) has been recommended as a reliable protein f or correction of the unknown dilution in tracheal aspirate samples from pre term infants. To investigate whether FSC would also provide a valid standar dization protein for samples of nasopharyngeal secretions, this study deter mined the intersubject variation and the alteration over time in the concen trations of FSC in nasal secretions from 35 children (median age 14 months) who participated in an antibiotic efficacy trial. Nasopharyngeal aspirates were obtained at enrolment and after 2-3 d. FSC in the specimens was quant ified by a direct enzyme immunoassay. The concentrations of FSC in the nasa l secretions ranged from 0.08 to 189.6 mu g ml(-1) (median 12.3 mu g ml(-1) ); the ratio of the highest to the lowest concentrations was 2370, the diff erence between the 90th and 10th percentile concentrations was 189-fold and the difference between the 75th and 25th percentile values was 26. FSC con centrations were significantly lower in children aged less than or equal to 12 months (median 2.2 mu g ml(-1)) than in the older children (median 21.5 mu g ml(-1); p = 0.035). Between the first and the follow-up specimens, 65 % of the children had greater than or equal to 2-fold difference in the lev els of FSC in the secretions. Because an optimal standardization protein sh ould show minimal variation between individuals and over time, FSC may not be a suitable protein for correction of the unknown dilution of nasopharyng eal specimens from children with upper respiratory tract infection.