2004, Volume 15, Number 2
By Danny L. Wiedbrauk, Ph.D.
Scientific Director, Virology and Molecular Biology
Research studies over the past 20 years have shown that certain human papillomaviruses (HPV) are responsible for nearly all cases of cervical cancer. Because of these findings, HPV testing has become an increasingly important gate-keeper in the cervical cancer screening and detection algorithms in the United States.
With the increased importance of HPV testing, there has been some concern regarding ThinPrep specimens that are canceled by Warde Medical Laboratory because the specimen volume was not sufficient (QNS) for HPV testing. Studies at Warde and other laboratories have shown that most of these short-volume specimens have low cell concentrations. Starting cell concentration is important because the ThinPrep 2000® and the ThinPrep 3000® processors use pressure/flow-rate sensing devices to collect the correct number of cells for each slide. In this process, the sample liquid is drawn through ThinPrep filter and the cellular material is trapped on the membrane. The trapped cells effectively clog the filter and decrease the flow-rate. As the number of cells on the filter increases, the flow rate decreases until a target rate is achieved. At this point, the filter contains sufficient cells to produce a high quality slide.
When specimens have low cell densities, the ThinPrep system must filter a larger fluid volume before enough cells are captured to produce a slide. Specimens with low cell densities are often QNS for HPV testing.
Several laboratories have asked if they can add more PreservCyt® solution to the vial to make up the volume difference. This approach is not appropriate because HPV is cell associated. Specimens with low cell concentrations will have most of the cells removed by the ThinPrep instrument. Adding more PreservCyt® solution to the vial in order to make up short volumes can produce a false-negative result by reducing the cell concentrations even further.
Good specimen collection and cell release techniques are crucial in order to provide a high quality ThinPrep specimen that has sufficient cellularity for liquid PAP and HPV testing. Physicians should be reminded to:
- Collect an adequate specimen using a broom-like collection device,
cervical brush, or a plastic spatula. Note:
Wooden spatulas should not be used because cells stick tightly to the
wood and they are difficult to dislodge into the PreservCyt® fluid.
- Rinse the collection device vigorously to dislodge as many cells as possible. The broom-like device should be forced against the bottom of the vial 10 times and the cervical brush should be rotated 10 times against the inside of the vial to release any cellular material.
ThinPrep Quick Reference Guides for broom-like devices, endocervical
brushes, and spatulas can be obtained from Cytyc Corporation [ (800) 442-9892].