If one kidney is small and the other one is completely normal, it is usually possible to lead a normal life without problems. However, if both kidneys are small, there may be kidney failure. Also, the one small kidney can cause problems such as high blood pressure, even if the other kidney is normal. Therefore people who are found to have a small kidney normally have some medical tests performed to see whether further treatment or observation over a period of time is necessary.
An infection in a kidney can cause it to shrink. Normally kidney infections do not cause permanent damage to a kidney, or leave a small scarred area in the kidney. Occasionally, though, a severe kidney infection (acute pyelonephritis) can damage the kidney so much that it becomes small. An infection bad enough to cause this may occur in someone with reflux nephropathy.
Surgical removal of a small kidney is often performed, but is not always necessary. If a small kidney is causing no problems there is no need to remove it. If the kidney is causing pain or recurrent infection, or is suspected to be a cause of high blood pressure, removal may be indicated. Doctors are usually reluctant to remove a kidney that is doing useful work, even if it is causing some trouble, in case a problem develops with the better kidney in the future. If a small kidney is providing more than 25% (one quarter) of the total level of kidney function (this can be measured using a test called a radioisotope scan), doctors often suggest trying to control any problems caused by the kidney with drugs (such as long term antibiotics for infection), before removing the kidney.