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Pitfalls in colorectal cancer diagnosis

The MDU recently highlighted malignancy as the biggest area of missed diagnoses by GPs – Dr William Hamilton and Professor Deborah Sharp advise on how to minimise the risk and Professor David Weller outlines the evidence on screening

Key points

lThe rectal examination is the most important – at least one colorectal cancer in 10 is palpable

lDiarrhoea is a more common sign of cancer than constipation

l17 per cent of patients presenting with iron-deficiency anaemia have cancer

lDiagnosis is not easy, so if a patient thinks a symptom is important enough to discuss,

it probably is

A full-time GP will have roughly one patient each year diagnosed with a colorectal cancer. For some of these patients the GP will not be involved in making the diagnosis: this is because around 20 per cent present as a surgical emergency, where the GP's diagnostic skills are in recognising the bowel obstruction or, less frequently, perforation.

A smaller percentage – less than 10 per cent in the UK but over 20 per cent in the US – are diagnosed following a surveillance procedure, because they are perceived to be at increased risk from their past medical or family history.

Patients who received screening for bowel cancer in trials had better cancer staging, fewer emergency admissions and had a relative reduction of over 15 per cent in colorectal cancer mortality.

The logistics of screening are being piloted, with a Government commitment to its introduction nationally once the practicalities have been sorted out.

In this article we concentrate on the remaining three-quarters of patients who attend their doctor with symptoms. The GP has to decide whether these symptoms could be caused by a cancer. Theoretically this is easy: the symptom complexes suggesting colorectal cancer have been described in textbooks for years.

The Referral Guidelines for Suspected Cancer – sent to every GP in 2000 – outline the specific presentations that are high risk (see table, right). They even identify low-risk symptoms, such as constipation. Imminent NICE guidance on colorectal cancer services will add to the paperwork on the GP's desk. The truth of course is that it's much harder than any guideline suggests.

This is not to blame guidelines: by their very nature they have to be simplistic. Patients don't have diarrhoea for six weeks. They have 'a bit of a loose tummy, which got better for a day or two, and then worse again after Arthur's birthday'.

We recently examined the evidence behind the recommendations1 in the referral guidelines and have investigated the primary care presentation of these cancers in 349 patients in the South West. From this and our experience as GPs, we have tried to identify hazards – or pitfalls – in the diagnosis of colorectal cancer.

Rectal bleeding

This is a classical symptom of colorectal cancer and a frequent first symptom. It is remarkably common in the community (see box, left). It is easier for the GP than most of the other symptoms, in that the list of possible diagnoses is short.

The few primary care studies that have been reported suggest that rectal bleeding the patient deems serious enough to report to their GP comes from an underlying cancer in about 3 per cent of patients.

The percentage rises with age. One danger inherent in the guidelines is the notion that local symptoms such as soreness, itching or lumps at the anus change rectal bleeding from high risk to low risk. Many of us know of patients whose rectal bleeding is ascribed to haemorrhoids, only to return months later with advanced bowel cancer.

The rectal examination is the most important examination of all. At least one colorectal cancer in 10 is palpable rectally.

It may be the true proportion is much higher as rectal examinations are unrecorded in the notes of the majority of colorectal cancer patients: while this may be a recording issue (quite a different, but also important problem!) it probably reflects a genuine under-examination of patients with possible cancer.

A further pitfall is the common belief that bright red bleeding always signifies a condition in the rectum or anus. However, it has frequently been described with caecal cancers. Therefore, a negative rectal examination and proctoscopy is not enough to exclude a tumour in this scenario.

Change in bowel habit

This is a real diagnostic struggle for primary care. The simple problem is that complaints of diarrhoea and constipation are common, yet every GP knows these symptoms just might represent a bowel cancer.

Not surprisingly, cancers presenting with diarrhoea have the longest symptom-to-diagnosis interval of all. Are there any clues? Some – but none of them watertight.

First, the standard teaching is that a patient who rarely consults, and then reports a symptom, is much more likely to have something seriously wrong. An elegant study by Dr Nick Summerton showed this was also true for cancer2.

Second, we have to be careful when diagnosing irritable bowel syndrome: in our case series one in six cancer patients had been labelled with IBS before their diagnosis, while only one in 40 of the non-cancer population had this label.

Of course IBS exists, but we need to keep an open mind when giving this label: would it harm to check a full blood count?

Third, if there is nothing abnormal on culture of a stool sample it is still important to look at the microscopy result.

Red blood cells seen in the stool are a strong pointer towards cancer. Constipation is a much less common harbinger of cancer, with the main concern being the possibility of an early obstruction.


Like diarrhoea, iron-deficiency anaemia is associated with considerable delays in diagnosis.

In part this is because it generates symptoms of its own, and symptoms from the GI tract – always supposing there are any – may be minor in comparison.

Of course as GPs we know that we need to exclude bowel cancer when a patient has anaemia: somehow it doesn't always happen.

In an elegant study of primary care management of anaemia3, Logan and colleagues found less than half of patients were adequately investigated, yet 17 per cent transpired to have cancer.

The problem is that the different presentations of anaemia may lead to non-GI referrals: we've seen neurology referral (faintness), geriatric referrals (weakness), and cardiology referrals (palpitations) in our series. Even haematology referrals may not be the best route to choose.


Diagnosis of colorectal cancer is not easy, particularly when our patients are human beings with each person having a different threshold for presenting symptoms to their doctor. But that may be a strength too: if a patient thinks their symptom – like rectal bleeding – is important enough to discuss, then it probably is.

The probability of a symptom meaning the patient has cancer progressively rises from the symptom in the general population to the symptom in the GP's surgery, and to the symptom in the GI surgeon's clinic.

The biggest of these rises in probability is when the patient chooses to bring the symptom to the GP. So patients are better diagnosticians than doctors? Well, maybe...

William Hamilton is research fellow and Deborah Sharp is professor of primary health care at the division of primary health care, University of Bristol

Rectal bleeding: the full picture

•2000 population

•280-660 have rectal bleeding sometime in their lives

•40-200 have bleeding in the last year

•14-30 report it to their GPs

•One has cancer

Summerton et al, 20032

Department of Health referral guidelines for suspected cancer

Colorectal cancer: guidelines for urgent referral

It is recommended these symptom and sign combinations when occurring for the first time should be used to identify patients for urgent referral under the two-week standard

Age threshold

•Rectal bleeding with a change in bowel habit to looser stools all ages

and/or increased frequency of defecation persistent for six weeks

•A definite palpable right-sided abdominal mass all ages

•A definite palpable rectal (not pelvic) mass all ages

••Rectal bleeding persistently without anal symptoms* over 60†

•Change of bowel habit to looser stools and/or increased over 60†

frequency of defecation, without rectal bleeding and

persistent for six weeks

•Iron deficiency anaemia without an obvious cause

(Hb<11g l="" in="" men="" or=""><10g l="" in="" postmenopausal="">

NB Patients with the following symptoms and no abdominal or rectal mass are at very low risk of cancer

•Rectal bleeding with anal symptoms*

•Change in bowel habit to decreased frequency of defecation and harder stools

•Abdominal pain without clear evidence of intestinal obstruction; anal symptoms include soreness, discomfort, itching, lumps and prolapse as well as pain

*Anal symptoms include soreness, discomfort, itching, lumps and prolapse as well as pain

†60 is considered to be the maximum age threshold. Local Cancer Networks may

elect to set a lower age threshold, such as 55 or 50 years


1. Hamilton and Sharp. Diagnosis of colorectal cancer

in primary care: the evidence base for guidelines.

Family Practice 2004;21:99-106

2. Summerton et al .The general practitioner-patient consultation pattern as a tool for cancer diagnosis in general practice. Br J Gen Pract 2003;53:50-52

3. Logan et al. Investigation and management of iron deficiency anaemia in general practice: a cluster randomised controlled trial of a simple management prompt. Postgraduate Medical Journal 2002;78:533-537

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