In 99mTc Sulfur colloid scan, a colloid shift refers to the abnormal biodistribution of radiotracer.
In a normal sulfur colloid scan, 80-90% of the injected colloid particles end up in the liver. The majority of the remainder localizes to the spleen. What little is left, ends up in the bone marrow, but usually in insufficient amounts to show up on imaging.
When this normal biodistribution is disrupted, we get increased uptake of radiotracer in the spleen and bone marrow compared to the liver.
The most common causes of colloid shift are hepatic cirrhosis, diffuse hepatic metastases, diabetes, and blunt trauma to the spleen (Briggs and Amberson, 1987).