The Pap smear has been practised for more than 80 years around the world as a conventional cervical cancer screening approach. But the classic test has certain limitations.
"A Pap smear is inexpensive. Not just doctors but also nurses are qualified to carry out the procedure. But one significant downside is that the result is not highly accurate," explained Dr Wisit Supakarapongkul, gynaecologist and president of the Thai Gynaecologic Cancer Society.
Involving a medical practitioner taking a small sample of cells from the surface of the cervix, putting them on a glass slide, staining them and checking through a microscope, the Pap smear has been commonly performed to detect if cells are infected with human papillomavirus or HPV — the major cause of cervical cancer.
Every country that has established the Pap smear as a screening protocol sees a 70% reduction in chances of developing cervical cancer — the world's fourth most common cancer in women, according to the International Agency for Research on Cancer, and Thailand's second most common cancer in females after breast cancer.
But according to Dr Thomas Wright, professor emeritus of pathology and cell biology from Columbia University Medical Centre in the United States, one of the noteworthy limitations to such cervical cytology is the fact that the Pap smear is admittedly not a very sensitive test, meaning it can frequently miss the disease especially in the very early stages, which usually shows no symptoms of cancer and as a result, women have to undergo the test quite often.
"One third of Pap smear tests have missed the disease," noted Dr Wright. "Also the test does not identify women at risk of cervical cancer."
Amid those flaws of the most familiar cervical cancer screening approach comes new medical screening technologies, which are said to be able to detect the likelihood of cervical cancer at the DNA level.
"Many countries start switching [from Pap smear alone] to HPV DNA testing, which is able to identify women at risk [of developing cervical cancer] and is much more sensitive. Instead of missing one third of cervical cancer cases, this new test only misses 5%," Dr Wright added.
On the surface, the process of performing the HPV DNA test looks pretty much similar to a Pap smear with a medical practitioner taking cells from the cervix. But instead of rendering unclear results, this new technique uses a sample of cervical cells to identify 14 high-risk types of HPV — specifically HPV 16 and HPV 18 — even when women have no cancer symptoms. In other words, the test can detect the risk of pre-cancer even before changes appear in the cells of the cervix. If the HPV DNA test detects either of these two HPV types, it is recommended that the patient get a colposcopy — a more detailed examination of the cervical cells.
Another new screening breakthrough is called p16/Ki67 dual-stain examination, which is said to be able to identify the presence of transforming HPV infection.
The test is performed on the same cervical sample collected for pap cytology and detects two biomarkers — p16 and Ki67 — whose respective functions are to tell cells to stop proliferating and to grow. In case high levels of both p16 and Ki67 are found in the same cervical cell, it means that something is wrong and that particular cell is transformed by HPV and further procedure is then required.
"This dual-staining technique has been used in a number of clinical situations," Dr Wright explained about the sensitivity of the new test. "If we use HPV for screening, we find about 10% of women who are HPV positive. Most of those women do not have the disease so one way we use to try to figure out which of the women has a high-grade disease is we look for HPV 16 and 18. Another way we tell which women have a high-grade disease is to do a Pap test on them and to do dual-staining. If we do that, we have found that about 88% of women with high-grade disease show positive results by the dual stain.
In the United States and some European countries, a conventional Pap smear is carried out together with the HPV DNA test to ensure that the result is accurate. In Thailand, according to Dr Wisit, the test will soon be available. HPV DNA in Thailand will cost approximately 3,000 to 5,000 baht depending on healthcare providers. Results in most cases are ready within a day.
And besides HPV vaccines, screening is regarded as the most effective shield against cervical cancer. The Royal Thai College of Obstetricians and Gynaecologists has recommended women above the age of 25, who have had sexual intercourse, and women above the age of 30, but have never had sex, undergo a Pap smear every two years. But in the not-so-distant future when the HPV DNA test is available, the examination is advised for women above the age of 30 and can be performed once every three years.
"One important thing to note is that screening is not a diagnosis," remarked Dr Wisit. "Screening tests are only to help medical specialists plan ahead in terms of further diagnosis and treatments. So we can never underestimate the importance of cervical cancer screening."
