Testing Cytyc® ThinPrep® Specimens for Chlamydia trachomatis and Neisseria gonorrhoeae: Problems in Paradise?
The laboratory community has long understood that a unified collection system for molecular infectious disease testing has a number of virtues including physician convenience, reduced storage space requirements in physician offices, and allowing add-on molecular testing without collecting a new specimen. To this end, Cytyc Corporation, in conjunction with several clinical laboratories have been aggressively marketing the use of Cytyc’s ThinPrep® Pap Test™ collection vial for liquid PAP, human papillomavirus (HPV), Chlamydia trachomatis, and Neisseria gonorrhoeae (GC) testing. Despite an increasing interest in this type of testing, Warde Medical Laboratory does not test ThinPrep specimens for chlamydia and GC due to technical concerns regarding the ThinPrep instruments and procedures.
False Positive Reactions.
The ThinPrep cytology instruments were designed to safely and effectively handle cells, not nucleic acids. Although a separate cylindrical filter is used with each specimen, the fluid in the ThinPrep vial is manipulated via air pressure. Air is bubbled through the fluid to evenly distribute cells, suction is used to draw the fluid onto the filter, and a blast of air is used to transfer the cells from the filter onto the slide. All of these activities can generate DNA-laden aerosols that can contaminate subsequent specimens. Aerosol contamination is a well-known problem in molecular diagnostic laboratories and these facilities employ special engineering and design features, laboratory practices, and monitoring activities to minimize aerosol contamination in all of their activities (1-3). These aerosol control features are not incorporated into the ThinPrep cytology instrument.
Aerosol generation is not a trivial issue because the transfer of just one nucleic acid molecule can produce a false-positive reaction in PCR and other nucleic acid amplification procedures. False positives are a special problem when testing for Chlamydia trachomatis and Neisseria gonorrhoeae (GC) because they can have enormous family and social repercussions.
Poor Turnaround Times.
Specimens sent to the laboratory in ThinPrep vials are usually sent to the Cytology laboratory for PAP smear analysis before other testing is performed. This routing was established because reflex testing for HPV is now recommended for managing patients with a cytological diagnosis of “atypical squamous cells of undetermined significance” (ASCUS) (4). When PAP testing is ordered on a ThinPrep specimen, it can take 1-3 weeks for the ThinPrep vial to arrive in the Molecular Biology Laboratory. A 1-3 week delay in receiving chlamydia and GC results is usually not acceptable to the practicing physician. We therefore recommend collecting a second specimen if chlamydia and GC testing are required.
It has been argued that false positive results and poor turnaround times could be minimized if the ThinPrep specimen was directed to the molecular biology laboratory before it was sent to the cytology laboratory. While this argument has some merit, it should be noted that sequential testing of a single specimen increases the likelihood of specimen DNA contamination, specimen handling problems, and could result in insufficient specimen volumes to perform the ordered tests.
False Negative Reactions.
The use of ThinPrep vials may produce false negative reactions due to specimen dilution. This is more likely to happen when testing asymptomatic patients because these patients often have low chlamydia and/or GC loads. ThinPrep vials contain nearly 7 times more fluid than the Aptima Combo 2 transport tubes that are supplied by Warde Medical Laboratory. Cervical samplers that are swirled in the ThinPrep PreservCyte® transport fluid will produce chlamydia and/or GC concentrations that are 7-fold lower than concentrations produced when cervical swabs are immersed in Aptima transport medium. This dilution-effect could produce false negative results in some patients. The laboratory could conceivably extract 7 times more PreservCyte fluid and thereby, improve the assay sensitivity. However, larger scale extractions are more expensive and they can deplete the PreservCyte fluid volume so that other tests cannot be done on the specimen.
ThinPrep specimen vials cannot be used for urine specimens.
On June 6, 2002 Cytyc Corporation announced that the U.S. Food and Drug Administration (FDA) had approved testing for Chlamydia trachomatis and Neisseria gonorrhea directly from the ThinPrep® Pap Test™ collection vial using Roche Diagnostics Corporation’s COBAS AMPLICOR™ automated system (5). It should be noted, however, that FDA approval was issued to Cytyc, not Roche Diagnostics. The AMPLICOR product insert makes no claims regarding the use of the Cytyc product in their assay. Indeed, instructions for processing and testing PreservCyt specimens using the Roche procedure must be obtained from the Cytyc Corporation. At first glance, this appears to be a minor inconvenience. Closer examination, however, reveals an underlying problem in that Roche has no obligation to make their product work with PreservCyte specimens. There is no guarantee that future enhancements to the Roche protocol will allow the assay to perform properly with PreservCyt specimens.
For the reasons stated above, Warde Medical Laboratory does not perform testing for C. trachomatis and N. gonorrhoeae on ThinPrep specimens. Questions regarding this policy should be directed to Dr. Dan Wiedbrauk or to Dr. David Keren at 800-876-6522.
- Wiedbrauk, D.L., and J. Stoerker. 1995. Quality Assurance in the Molecular Virology Laboratory. p. 25-37. In: Wiedbrauk, D.L., and D.H. Farkas (eds.), Molecular Methods for Virus Detection. Academic Press, San Diego, CA.
- Sikro, D. A. and G. D. Ehrlich. 1993. Laboratory facilities, protocols, and operations. p. 19-43. In: , Ehrlich, G.D. and S. J. Greenberg, S.J., (eds), PCR-based diagnostics in infectious disease. Blackwell Scientific Publishers, London, England.
- McCreedy, B. J. and T. H. Callaway. 1993. Laboratory design and work flow. p. 149-159. In: Persing, D. H., T. F. Smith, F. C. Tenover, and T. J. White (eds.), Diagnostic molecular microbiology: Principles and application. American Society for Microbiology, Washington, DC.
- Wright, T. C. Jr., J. T. Cox, L. S. Massad, L. B. Twiggs, and E. J. Wilkinson. 2001. Consensus guidelines for the management of women with cervical cytological abnormalities. JAMA 287(16):2120-2129.
- Cytyc Press Release. http://ir.cytyc.com/ReleaseDetail.cfm?ReleaseID=101474