The idea of a cervical smear is to detect pre-cancerous cells so that they can be removed before they become dangerous.
A doctor or nurse using a spatula takes a scraping of cells from the cervix (neck of the womb) and "smears" the cells in a thin layer on a glass slide. After staining with a 'Pap' stain the cells are examined by a cytologist in a laboratory who will check for signs of pre-cancerous change. Each slide contains between 300,000 to 400,000 cells and takes about 10 minutes to scrutinise.
A new 'Pap' smear technique such as "ThinPrep", which originated in the USA and is now available in the UK, involves placing the cells in a small vial of preservative rather than smearing them on a slide. The vial goes to the laboratory where a high quality slide is prepared after impurities, such as red and white blood cells, are filtered out. The cells in the preserving fluid can also be tested for human papillomavirus (HPV) to look for high risk sub-types.
The doctor or nurse will try to obtain cells from the cervix where the skin (squamous) cells on the outside meet the gland (columnar) cells on the inside of the canal. This junction is called the Transformation Zone (TZ) and it is here that cancers most often start. A good quality smear will therefore contain both sorts of cells to prove that the TZ has been sampled correctly.
Five to ten percent of all smears may be inadequate or unsatisfactory, usually because there are not enough cells on the slide for a comprehensive report. The cells may be obscured by blood if the smear was taken near to a period or covered in debris and white blood cells if some inflammation is present. In these cases, the smear is repeated, sometimes after any infection is treated.
Rarely, the smear will contain cells which suggest that precancerous changes have already progressed into cancer, or that there is disease in the glandular lining of the cervical canal, and urgent investigations are required.
The cytologist examining the slide will look for pre-cancerous changes in the cells known as dyskaryosis or dysplasia. There are four types:
In borderline smears there is a slight abnormality of the cells and their nuclei. A repeat smear after an interval of around 6 months is usually requested. In the United States borderline changes are known as 'Atypical Squamous Cells of Uncertain Significance' (ASCUS).
Essentially, the nucleus in the centre of the cell gets bigger and the jelly around the nucleus (cytoplasm) gets smaller. These changes are sometimes caused by certain strains of the Human Papilloma Virus (HPV) or "wart" virus.
About 50% of women with mild dysplasia will have their changes revert to normal if you wait 6 months. So women with a smear showing mild changes will be asked to have a repeat smear 6 months later. If mild changes persist on the second smear, those women will undergo further assessment by colposcopy. Mild dyskaryosis in the United States is known as a 'Low-grade Squamous Intraepithelial Lesion' (LSIL). Women who have mild dyskaryosis and carry HPV 16 or 18 may well be offered treatment to remove the cells and those who do not carry HPV 16/18 might well be offered surveillance only as these cells may revert to normal given time
Women with smears showing moderate or severe pre-cancerous changes will be referred for colposcopy as they have a significant risk of proceeding to cervical cancer if left untreated. Moderate or severe dyskaryosis is known in the United States as a 'High-grade Squamous Intraepithelial Lesion' (HSIL).
Cells showing pre-cancerous changes show no special features to the naked eye. A cervix which contains these cells will look normal. Pre-cancerous cells do not cause any symptoms, and may remain dormant for several years before proceeding to cancerous change i.e. start nibbling into the tissue beneath.
Occasionally, cervical/Pap. smears show up infections such as candida (thrush) or trichomonas (TV). However, smears are not used to look for infections. They are used to screen large populations of women for signs of pre-cancerous changes.
Sometimes, red or white blood cells obscure the cells on the smear and may be reported as "bloodstained" or "inflammatory" respectively.
HPV ("wart virus") changes may also result in inflammatory or unsatisfactory smears and the cytologist may request a repeat smear to be taken at a later date.