Investigations
1st investigations to order
computed tomography
Test
Order an urgent computed tomography (CT) scan of the abdomen and pelvis to be performed as soon as possible.[2][6][24] Guidelines on acute abdominal pain from the Association of Surgeons of Great Britain recommend doing this within 24 hours.[3] However, in practice, CT for suspected large bowel obstruction should be performed sooner than that. Prompt radiological diagnosis helps to ensure admission to the correct specialty and avoid delays in treatment.[23]
Consider CT with intravenous contrast, according to local hospital protocols.[23]
Check the patient’s kidney function, as acute kidney injury may be a contraindication to a CT scan using a contrast agent.[23][34]
CT can be used to identify perforation and dilatation and stage malignant disease.[3][5] The detail that a CT scan provides with regard to the severity and aetiology of the obstruction will help to establish the management plan.
Use the CT scan to identify the level and cause of the obstruction, and therefore determine the management plan.[23]
Use CT to differentiate between obstruction and pseudo-obstruction.[24]
CT evidence of a non-adhesional cause (tumour, hernia, or volvulus) or evidence of bowel ischaemia is an indication for surgery.[3]
CT can show whether the ileocaecal valve is patent.
With gas trapped inside and continuing to enter the colon, pressure within the large bowel will continue to increase and cannot flow back into the small bowel. This is known as closed loop obstruction and is a surgical emergency. It leads to a rapid increase in distension and intraluminal pressure, with early vascular occlusion. It can cause ischaemia and result in perforation.[1]
In particular, perform a CT scan if the patient is over 50 and presenting with abdominal pain, due to the risk of occult malignancy in this group.[3]
Practical tip
Plain radiography
Do not routinely use plain radiography of the chest and abdomen. It is unreliable diagnosis usually requires further imaging with CT; and should only be used when CT is unavailable.[38] Abdominal radiographs can be extremely difficult to interpret. Consult with radiology.
Plain radiography may be the imaging that the patient is first sent for, but most hospitals are reducing its use in favour of CT.
Result
May visualise obstruction, perforation, dilatation, ischaemia, malignancy; evidence of a non-adhesional cause (tumour, hernia, or volvulus).
In acute diverticulitis, CT scan may show localised inflammation, abscess formation, perforation, obstruction of fistulation into the bladder or vagina.[3]
Excludes pseudo-obstruction.
full blood count
Test
Check white blood cell count, haematocrit, and for iron deficiency anaemia.
Urgently investigate and refer men and non-menstruating women with iron deficiency anaemia, as 10% of such patients will have colorectal cancer.[19]
Result
WBC >10,000/mm3 is a non-specific marker of inflammation; it may indicate bowel perforation or necrosis.[5]
Neutrophilia may indicate bowel perforation or necrosis.[5]
A low haematocrit may indicate blood loss caused by an underlying disease. Occasionally blood can be lost into an obstructed bowel, a potential sign of intestinal necrosis.
Iron deficiency anaemia may be a sign of colorectal cancer.[19]
electrolytes
Test
Electrolyte imbalances are often seen in patients with bowel obstruction – in particular, low potassium. This may be caused by fluid loss into the obstructed bowel or through vomiting.
Patients on renal dialysis can have symptoms of ileus due to electrolyte imbalance.[39]
Result
Deranged electrolytes; in particular, low potassium.
C-reactive protein
Test
Check for elevated C-reactive protein (CRP).
Result
Elevated CRP (>75 mg/L) may indicate inflammation.
urea/creatinine ratio
Test
Dehydration may result in acute kidney injury.
Result
Elevated levels suggest dehydration/hypovolaemia and increased risk for development of severe disease.
glucose
Test
Check blood glucose level.[3]
Diabetic ketoacidosis (DKA) can present with abdominal pain.[40] DKA consists of the biochemical triad of ketonaemia (ketosis), hyperglycaemia, and acidaemia.
See our topic Diabetic ketoacidosis.
Result
Blood glucose >11 mmol/L, as one of the triad of biochemical signs, indicates DKA.[41]
clotting, group and save, or cross-match
Result
Consult your hospital guidelines or consult with haematology.
arterial blood gas (including lactate)
Test
Request in all patients to obtain a lactate reading.
Result
An elevated lactate reading indicates poor tissue perfusion. It is not diagnostic for intestinal ischaemia.
Lactic acidosis may indicate perforation or necrosis.[5]
serum amylase/lipase
Test
Can be elevated with any significant intra-abdominal event.
Result
elevated
Investigations to consider
beta-human chorionic gonadotrophin
Test
Urine or serum beta-human chorionic gonadotrophin (hCG) in women of childbearing age.[3]
Result
Consult with gynaecology if positive.
urinalysis
ultrasound
Test
Ultrasound has been shown to have a comparable accuracy to CT scans in the detection of small bowel obstruction but diagnostic criteria and accuracy for large bowel obstruction are less well defined.[35] One proposed criteria include dilated large intestine (>4.5 cm) and presence of abdominal A-lines (gastrointestinal intraluminal air).[36]
Result
Dilated large intestine (>4.5 cm)
water-soluble contrast study
Test
Consult radiology if considering water-soluble contrast study.
It is not frequently used in most UK hospitals. Water-soluble contrast study is less sensitive than CT for identifying perforation of the bowel.[3]
Compared with plain radiography, the addition of contrast (given to the patient as an enema) to the plain radiograph improves the diagnostic accuracy in suspected large bowel obstruction.[37]
Result
Presence or absence of contrast in the colon on abdominal x-ray.
lower gastrointestinal endoscopy or flexible sigmoidoscopy
Test
Use lower gastrointestinal endoscopy or flexible sigmoidoscopy to confirm a diagnosis of mechanical obstruction or to decompress a volvulus or pseudo-obstruction.[9] However, this can introduce more gas into the large bowel and increase the distention in the case of a stricture.
Result
Normal luminal mucosa or non-viable mucosa.
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