Liquid-based Cytology – Test of choice
Cytology is the study of cells. Just over half a century ago, it was widely recognised and accepted that cells shed or sampled from the surface of the cervix reflected changes in the cervical epithelium, allowing for the detection of abnormal cells that indicated preinvasive or invasive cervical disease. Since then, cytology has been used in many countries as the major method of screening for cervical cancer. For years, the method involved collecting a sample of cells from the cervix, which was then spread directly onto a glass slide that would be immediately fixed in alcohol or with a spray fixative. This method is simple to perform but has certain drawbacks. Occasional smears are thick or show airdrying artefact or obscuring elements such as blood or inflammatory exudate. These make the smear difficult to evaluate or even unsatisfactory for assessment. Liquid based cytology is a method that has been developed to overcome some of these drawbacks.
Liquid-based Cytology
Basically, in liquid based cytology, the cell sample from the cervix is rinsed into a vial of liquid fixative instead of being spread onto a glass slide. Depending on the system, the sampling device may be rinsed in the vial and then discarded, or the sampling device may be detached and left in the vial. The vial is then sent to the laboratory where it is processed to make a smear. The two most widely used technologies utilise either a filter-transfer method or a density gradient method. KK Women’s and Children’s Hospital has elected to use the ThinPrep system as its liquid based cytology test.
ThinPrep system
The ThinPrep system is a filter transfer method. In this system, the sampling device is rinsed thoroughly in the liquid fixative and then discarded. The vial is sent to the laboratory. In the laboratory, the vial is placed into a processor (the machine which prepares the smears) together with a glass slide and filter mechanism. The processor immerses the filter assembly into the vial and spins it at a high speed to ensure an even mix of the cells and to break up large cell groups. The fixative is then sucked through a filter membrane which traps the cells but allows fluid through. When an adequate number of cells have been deposited on the filter, the processor detects a drop in the suction pressure and stops drawing fluid through the filter. It then applies the filter to a specially prepared glass slide and transfers the cells across. The slide is deposited into a vial of fixative from which it is subsequently taken out and stained.
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Comparison between Thin Prep smear and a conventional smear
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Laboratory Assistant processing the Liquid Based Sample into a smear with Thin Prep T2000 processor
Advantages
There are several advantages to this system. One is that it produces a thin layer of cells which is easier to evaluate than a thick smear. The cells are also better. In addition, the entire cell sample is captured in the fixative vial which leads to a more representative smear being prepared.
One of the most important advantages of this test is that the material that is left over after a smear has been prepared, can be used for adjunctive testing; The best known of these tests are those for HPV DNA. Under certain circumstances, a doctor may feel that knowing the HPV status of a woman with abnormal cells on cytology will help in decisions on management. With liquid based cytology, the left over material can be sent for testing (also known as reflex testing) without having to recall the patient, saving the patient time and trouble. The specimen can also be used to test for certain microorganisms.
A further advantage is that the smears may be initially subjected to image analysis. Computer software “reads” the smears and registers the co-ordinates of the fields with what it regards as the most abnormal cells. On review the system directs the cytotechnologists to these fields where they are evaluated. This can cut down on technologists’ screening time.
Disadvantages
However, there are also some disadvantages, which include increased manpower needed to prepare the smears, and the dependence of smear preparation on the instrument.
Test of Choice
Nevertheless, liquid based cytology is being increasingly adopted worldwide as the test of choice for cervical cytology and will no doubt, in future, further demonstrate its utility in allowing a spectrum of adjunctive tests to be performed on residual material.
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Need indepth information ?
Access our Conditions & Treatments sections for related topics on Cervical Cancer.
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