A Synopsis of AFP's "When to Order Contrast-Enhanced CT"

When to Order Contrast-Enhanced CT
JAMES V. RAWSON, MD, and ALLEN L. PELLETIER, MD, Medical College of Georgia at Georgia Health Sciences University, Augusta, Georgia
Am Fam Physician. 2013 Sep 1;88(5):312-316.
http://www.aafp.org/afp/2013/0901/p312.pdf

     CT imaging is a common test ordered these days. The two contrast agents usually used are barium and iodine. They can be given orally, rectally, or IV. Oral is used to check out the bowel. Iodine contrast is water based and is also given orally. It is not nephrogenic, but may rarely cause an aspiration-induced pulmonary edema (this risk can be minimized if the contrast is given through an NGT). Rectal contrast is used if the clinician suspects that there is a penetrating colonic injury. Intrathecal contrast is used in patients with possible CSF leaks and spinal/ cisternal disease.
     IV contrast is used if the clinician wants to look at vascular structures, abdominal, and pelvic organs. Nonionic contrast has a lower rate of adverse reactions, such as asthma and drug allergies. Although patients are always asked if they are allergic to shellfish before getting iodine contrast, there is no cross-reactivity between the two. Patients who have had a previous anaphylactic reaction (to anything) are at higher risk for a reaction to the contrast. A mild reaction need only be observed. A moderate reaction may need prompt treatment. A severe reaction may require rapid intervention and/or hospitalization. Patients with previous minor reactions may only need pretreatment with steroids and benadryl.
     Patients with renal insufficiency should have their creatinine checked. Those with measurements above 1.5-2 mg/dL, or those with a level that is going up, should be considered for an alternate form of contrast. Patients with normal renal function do not need their creatinine checked.
      Iodine contrast can cross the placenta and although it has not shown to cause any teratogenic or mutagenic effects, it should only be used when the results will affect the care of the fetus, or when the test cannot wait until after the pregnancy (and if other contrast/ imaging is inappropriate).
     Patients who are being treated for thyroid disease with iodine 131 should not have iodine contrast because it will make the treatment ineffective. The contrast should be given two months prior to the thyroid treatment, otherwise a endocrinologist referral is warranted.
     Patients who are on metformin who receive IV contrast will have an increased risk of metabolic acidosis because of the impaired metformin clearance.  Patients on metformin with no renal impairment do not need to have their creatinine checked and can continue the contrast with the metformin. Patients with normal renal function but at least one comorbid condition should stop the metformin when the contrast is given. The metformin can be resumed after 48 hours as long as the patient is clinically stable. Creatinine does not need to be rechecked. Patients with known renal dysfunction should have the metformin stopped and it should be resumed only after the renal status have been reevaluated. High risk populations may be given sodium bicarbonate, hydration, acetylcysteine and smaller contrast volume to reduce risk.
     Non-contrast CT can be used in patients with suspected acute stroke, closed head injury, lung disease, chronic dyspnea, soft tissue swelling, infection, trauma, kidney stones, and spinal trauma. Contrast enhanced CT is indicated for acute appendicitis, cancer staging, diverticulitis, pancreatitis, and pulmonary embolism.