Breast Screening

False negative results

Breast cancer is the most common cancer in women, and mammography is the only screening method that can detect breast cancer early. However, mammography is not perfect. While early detection offers clear advantages, mammography also has potential disadvantages. These include the possibility of false positives, where something is seen on the mammogram but no cancer is found, and false negatives, where the mammogram may look normal even though breast cancer is present. This may lull the woman and her doctor into a false sense of security. A woman who receives a false negative result - one that shows there is no cancer when there really is - may delay seeking medical care even if she has symptoms.

False negatives occur in 1 of every 2,500 women screened (NHS Breast Screening Programme – "Helping you decide" leaflet July 2013) and occur more often in younger women than in older women because the breast tissue of younger women is denser. The size of the tumour, the rate of tumour growth, the level of hormones, such as oestrogen and progesterone, in the woman's body, and the skill of the radiologist can also affect the chance of a false negative result.

Some of the women we spoke with who attended for routine breast screening had false negative results. One woman explained that, after her first mammogram, she was recalled for further tests which all showed that she did not have breast cancer. Several months later, when she noticed 'something odd' in her breast, more tests showed that she did actually have cancer. Her cancer, however, had not been picked up by the radiologist who had read the first mammogram. She nevertheless encouraged women to attend for breast screening (see 'General views on screening').

Mammography may miss some cancers. Some breast changes, including lumps that can be felt, don't show up on a mammogram. One woman said that, although her first mammogram was clear, four months later she discovered a lump that turned out to be cancerous. At first she was upset that her cancer had not been detected four months earlier, but then accepted that some lumps do not show on mammograms.

Breast changes can be especially difficult to spot on the mammograms of younger women because their breast tissue is dense. The NHS Breast Screening Programme invites well women (those with no symptoms) over 47 to attend for breast screening. A GP may refer women who discover breast symptoms between three yearly screens or under the age of 47 to a breast clinic for further tests (see 'Diagnostic mammograms in the UK'). These tests can include mammograms, MRIs, ultrasound scans, core biopsies and fine needle aspiration (FNAC) (see 'Referral to a breast clinic').

A younger woman we spoke to had been screened routinely since the age of 40 because of a family history of breast cancer. She had been referred for a mammogram between routine screens because of a bleeding nipple. She had a breast lump that did not show up on a mammogram. Although she had felt 'something' in her breast at the time, she dismissed it as nothing because the mammogram and ultrasound results were normal. Several months later, when she felt a lump in her other breast, she was referred to the breast clinic again and this time tests showed that she had cancer. This woman also discussed the false sense of reassurance that her original, normal mammogram had given her and felt that women should understand the limitations of breast screening.

Two younger women, who were not yet eligible for routine screening on the NHS Breast Screening Programme, also discussed having false negative results on mammograms that were done to investigate breast lumps they had found. One of them said a mammogram, ultrasound scan, fine needle and core biopsy all showed that her breast lump was benign. After she chose to have it removed by surgery, however, she found out the lump had actually been cancerous. This woman then had to have treatment for breast cancer (see our Breast cancer section). The other explained that, before surgery, her breast lump had not shown up on mammograms and both she and her doctors believed it was probably benign. However, after having it removed by surgery, she and her doctors learnt that it was cancerous (see 'Breast screening and younger women').

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Last reviewed March 2016.

Last updated March 2016.


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