Cervical smears:Double-check this inaccurate test

If you’re a woman, your GP or gynaecologist undoubtedly press-gangs you into having a Pap smear test periodically to screen against cervical cancer.

In these tests, named after Dr George Papanicolaou, who developed it in 1941, a tiny tissue sample is scraped from your cervix, smeared and fixed onto a slide, and sent to a lab, which examines the sample for the presence of abnormal cells. If the result shows any sort of abnormality, you are referred for further diagnostic tests, usually a direct examination of the cervix (colposcopy) or a biopsy, but sometimes even treatment for cancer.

Over the years, the smear test’s reputation has been stained by a number of catastrophic errors. At Kent and Canterbury Hospital, for instance, more than 90,000 smears taken during 1990-1995 had to be rechecked after eight women died, following mistakes in reporting their results. The problem, according to Britain’s National Institute for Clinical Excellence (NICE), is that the test is still appallingly inaccurate.

NICE estimates that up to 13 per cent of smear-test results are false positives and 20 per cent are false negatives, when women with possible problems have a result that comes back ‘normal’. In other words, out of 1000 women screened, two who could have cancer will be given the all-clear. Other research has estimated a false-negative rate of as much as 60 per cent (BMJ, 1986; 293: 659-63).

In the US, the Centers for Disease Control and Prevention (CDC) recently warned against annual Pap smear testing because of the high rate of false positives, particularly with low-grade abnormalities, leading to potentially damaging treatment for symptoms which might have gone away if left alone (MMWR, 2000; 49: 1001-3).

Consequently, some countries, including the US and Switzerland, have introduced a new technology called ‘liquid-based cytology screening’, or ‘monolayer cytology’. Here, a sample is collected using a special spatula, the head of which is rinsed into, or broken off into, a vial of preservative. Thus, the entire sample is immediately preserved in a liquid.

Pilot studies in the UK have shown that LBC screening produces more accurate results, with only 1.6 samples considered ‘inadequate’, or unable to be read, compared with more than 9 per cent with traditional Pap smear samples.

Nevertheless, a recent French study has found the LBC to be even less reliable than the Pap (BMJ, 2003; 326: 733).

However, these questionable conclusions have not deterred the UK government. On 22 October, it announced that LBC would be ‘rolled out’ over the next five years to replace the conventional Pap smear, at a cost of some £10 million.

What can you do instead? At very least, according to the CDC, waiting three years (or five years, if you’re over 50) won’t affect mortality rates. If you do get a positive result, insist on a repeat smear from another lab before going down the more invasive route of biopsies-or worse.

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Written by What Doctors Don't Tell You

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