Comparison of Flocked and Rayon Swabs for Collection of Respiratory Epithelial Cells from Uninfected Volunteers and Symptomatic Patients
Significantly more epithelial cells were collected by flocked swabs than by rayon swabs in parallel nasopharyngeal and nasal swabs taken from 16 volunteers. Nasopharyngeal sampling of 61 symptomatic patients also yielded more cells by flocked than rayon swabs, providing better clinical specimens for diagnosis.
Acute upper respiratory tract infections are the most common cause of illness in children (3) and in the institutionalized elderly. Timely laboratory diagnosis may allow specific antiviral therapy, avoidance or discontinuation of antibacterial agents, appropriate institutional infection control measures, and improved surveillance. Rapid testing with direct fluorescent antibody (DFA) shortens the time to laboratory diagnosis for upper respiratory viruses, but sampling must maximize the collection of respiratory epithelial cells, since the sensitivity varies with the number of infected cells examined (7).
We reasoned that the quantitation of total and infected respiratory epithelial cells would measure the adequacy of an upper respiratory tract sample but could identify no reports of using quantitative epithelial cell yield as an outcome measure to ascertain the efficacy of sampling or to compare different swab designs. Our primary study objective was to compare total respiratory epithelial cell yield of two swab designs among volunteers and symptomatic patients. Our secondary objectives were to compare nasal versus nasopharyngeal swabs (NS and NPS, respectively) among volunteers and to compare the number and proportion of infected cells between swab types among symptomatic patients. Sixteen healthy volunteers were recruited from laboratory staff, and four operators were recruited from investigators and trained research personnel. We directly compared a new, flocked swab (Copan Diagnostics, Corona, CA) to our standard rayon swab (Copan Diagnostics) (Fig. 1). The flocked swab contains short nylon fiber strands attached to molded plastic, with a hydrophilic layer of nylon pile that results in the efficient collection and release of particulate matter.
Each volunteer was swabbed four times, using both swab designs in opposite nares in randomized order. The NPS was inserted at a distance equivalent to that from the nares to earlobe, and the NS was inserted 4 to 5 cm. Volunteers assessed discomfort from lowest to highest on a 100-mm visual analog scale (VAS). The VAS consists of a horizontal line, 100 mm long, with “no discomfort” at one end and “the worst imaginable discomfort” at the other. The volunteer marked the point in the line corresponding to their perception of discomfort. For analysis, the number of millimeters from the “no discomfort” line was measured.
Nurses sampled 61 patients seen at St. Joseph’s Hospital, Hamilton, Ontario, Canada, for respiratory symptoms in January 2005, as part of routine care, using either swab type (flocked or rayon) according to availability. Samples were chosen by stratified systematic sampling from all consecutive NPS submitted to the laboratory during the study month by the two swab types and three etiologic groups: influenza virus positive, respiratory syncytial virus (RSV) positive, or DFA negative (10 samples in each of the six strata for a planned total of 60; 11 samples were obtained in one stratum). Twenty-one were adults over the age of 15 years, and forty were children. Personal identifiers were removed. Patient NPS specimens were collected using either swab according to the availability of each type on each nursing unit. Both types were in use concurrently during the study period. All swabs were placed in Universal Transport Media (Copan Diagnostics) and coded to maintain blinding.
Swabs were processed identically, according to current DFA protocols. After vortexing for 20 s to release the cells, the swabs were discarded. The medium was centrifuged, the pellet was resuspended in 1 ml of buffered saline, and then 25 l of suspension was added to the wells on a glass slide. Slides were air dried, fixed, and stained with fluorescein isothiocyanatelabeled monoclonal antibody against seven respiratory viruses and negative control (Diagnostics Hybrids, Inc., Athens, OH). Two independent readers, blinded to swab type, used a fluorescence microscope at 400 magnification and recorded the number of fluorescent and nonfluorescent cells per high-powered field (hpf).
The mean cell yield and visual analog data were log transformed as required to improve normality and compared by using a two-tailed paired (for volunteer) or unpaired (for patients) t test. Cell yield results from symptomatic patients were also adjusted for virus and age group in a multivariable linear regression model (SPSS for Windows 11.5). A P value of 0.05 was taken as statistically significant.