 The
diagnostic process for colo-rectal cancer can be incredibly
uncomfortable — just the thought of an endoscope
inserted into one's intestinal tract is enough to make
most people cringe. For those who prefer a less invasive
look-see, fecal occult-blood tests provide a noninvasive
alternative that uses stool samples to check for the presence
of minute amounts of blood. But is this an adequate diagnostic
tool?
"Although fecal occult-blood testing
is the only available noninvasive screening method that
reduces the risk of death from colorectal cancer, it
has limited sensitivity," wrote Dr Thomas Imperiale
of the Department of Medicine at Indiana University
and colleagues in the December 23 edition of the NEJM.
A BETTER ALTERNATIVE
Their report claims that examining
fecal matter for 21 DNA markers associated with colorectal
cancer is a more sensitive screen than the Hemoccult
II blood test. However, neither noninvasive test stacked
up against colonoscopy in terms of detection sensitivity.
In the NEJM study, a whopping
4,404 people aged 50 or more who showed no symptoms
of the disease participated. These subjects were asked
to provide a stool sample that was later analyzed using
the two noninvasive tests. Just to be on the safe side,
they were also given a colonoscopy.
A subgroup of 2,507 people who
were subsequently diagnosed with invasive adenocarcinoma
or advanced adenoma were analyzed to gauge the success
or failure of the noninvasive tests. As well, lucky
folks who were polyp free or in whom the detected polyps
were not a concern were randomly chosen to evaluate
the outcome of the DNA and occult blood tests.
When it came to the detection of
the 31 cancers that had begun to spread further afield,
the fecal DNA test detected 16, for a sensitivity rate
just shy of 52%. This less than stellar detection rate
was still far better than the occult-blood test, which
clocked in with a paltry 12.9%.
In 71 other patients, the invasive
colorectal cancer was accompanied by benign tumours
and abnormal cell or tissue growth. The DNA screen detected
29 of these cancers (a sensitivity of about 41%), compared
with only 10 for the occult-blood test (a sensitivity
of just over 14%). Advanced neoplasia occurred in 418
people. The DNA test produced a positive result in only
76 of these folks (18.2%) while the blood test proved
positive in 45 people (10.8%).
The researchers reasoned that the
occult blood-test performed dismally because it only
picked up on the intermittent bleeding of cancerous
growths and the near absence of bleeding in precancerous
growths. In contrast, abnormal DNA is routinely shed
from precancerous and full-blown cancerous growths.
fewer FALSE NEGATIVES
Granted, the noninvasive detection
sensitivities were not astounding but the detection
accuracies — determined by comparison of negative
samples confirmed by colonoscopy — was 94.4% for
the fecal DNA test and 95.2% for the occult-blood test.
So, a negative result was likely to be true.
While neither noninvasive test
rivals the detection accuracy of colonoscopy, the easy-to-do
nature of the tests may encourage more people to be
screened for colorectal cancer. Indeed, while colorectal
cancer is the third leading cause of cancer in Canada,
behind lung and breast cancers, and over 16,000 Canadians
are diagnosed each year, the number of Canadians 50
or older who choose to be screened for the disease languishes
at around 40%.
At present, the cost of the test
is high, rivalling that of a colonoscopy. "The cost
of the test is what is going to limit its use," Dr Imperiale
told HealthDay. But, with declining costs and improved
sensitivity, "it may be a useful screening technique
down the line for average-risk people."
NEJM Dec 23, 2004;351:2704-14
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