One test for diabetes measures glycated hemoglobin. Hereditary spherocytosis can cause misleading results in this test.
In each red blood cell there are around 250 million molecules of hemoglobin (the oxygen carrying molecule). Glucose reacts with hemoglobin to make it glycated. The higher the levels of glucose that a red blood cell encounters, the more molecules of hemoglobin become glycated. Each red blood cell can be thought of as a little glucose meter, accumulating glucose over its lifetime.
In a blood sample there will be young red blood cells that have relatively little glycated hemoglobin and older red blood cells with more. But the average level of glycated hemoglobin across these different ages correlates to the average level of glucose over the previous months.
In uncontrolled diabetes, the level of glucose in the body is higher than normal, so more hemoglobin becomes glycated.
With hereditary spherocytosis, and other hemolytic anemias, red blood cells do not survive for the normal 120 days. This means that the population of red blood cells in a blood sample is younger and so will not have so much glucated hemoglobin.
As a result, someone with hereditary spherocytosis appears to have lower levels of glucose with this test and this could lead to a failure to diagnose or control diabetes.