Why does a cardiologist do fasting blood tests? is this routine?

Why fast for tests: Fasting blood tests are used to get an idea of what your lipid and glucose levels are. If your lipids are elevated on a fasting blood sample, you need dietary, k exercise and/or drug therapy to control their levels. If you fasting glucose is normal, that does not rule out diabetes. Another test called a hemoglobin a1c will give you a better idea of your have diabetes if it's elevated.

Yes: Fasting blood test is routinely done especially for blood sugar and lipid panel.