Haemolysis is the breakdown of red blood cells. The concentration of many substances is different inside and outside red cells. This answer is only about haemolysis in blood samples. Most biochemical tests on blood use serum, the liquid remaining after blood has clotted or plasma, the liquid in a sample to which anticoagulant had been added.

To obtain serum or plasma a blood sample is centrifuged to separate out the cells or clot. If the sample is haemolysed the serum or plasma will have a pink or red colour. Haemolysis can interfere with biochemical tests in two ways. The haemoglobin can interfere with the reactions used in the tests because it's colour is close to that of the reaction product or there can be a very different concentration of a substance inside and outside the red cells.

An example of the second type is potassium. It's often important to measure potassium and a small level of haemolysis will give high results. Most modern hospital labs measure haemolysis on every sample and suppress results of any affected tests. Old fashioned labs will visually inspect the samples after centrifugation and make a manual comment and/or alteration.

Haemolysis of blood samples is almost always due to unskilled treatment of samples before they arrive in the lab. Using small diameter needles, squirting blood into a sample tube too hard and harsh treatment of samples eg shaking instead of inverting to mix are common causes.