A venogram is a catheter-based test in which a special dye (contrast medium) is injected into a vein prior to an x-ray. The dye appears on x-ray, allowing a physician to see the shape, size and composition of a vein, including any possible obstructions.

The venogram is one of the most accurate tests used to diagnose deep vein thrombosis (DVT), a serious condition when blood clots form in the deep veins of the legs. However, Doppler ultrasound has largely replaced the use of venograms for DVT diagnosis because it is less invasive and less expensive. Venograms may also be performed for other purposes, including the evaluation of congenital venous defects.

The venogram is usually  performed in a hospital or clinic on an outpatient basis. It typically takes about 30 to 45 minutes, depending on the location of the vein. An intravenous (IV) line is inserted into the patient’s vein and the dye is slowly injected through a catheter into the vein. The patient may feel a warm or flushing sensation when the dye is introduced. A series of x-rays will be taken as the dye travels through the vein to detect any abnormalities. The injection site may be sore for several days after the procedure but most patients can resume their normal activities without difficulty.

Patients should tell their physician if they have ever had an allergic reaction to shellfish, iodine, foods containing iodine or a contrast medium. The most severe side effect of the test is an allergic reaction, which includes signs such as flushing and breaking out into hives, and other symptoms such as shortness of breath and chest or throat constriction.