A cholesterol test reports several values, and the popular framing of good and bad cholesterol is a simplification that loses most of the useful information.
What is being measured
Cholesterol travels in the blood inside particles called lipoproteins. The test measures how much cholesterol is carried by different types.
LDL carries cholesterol to tissues and contributes to arterial plaque. Higher values are associated with higher cardiovascular risk, and this relationship is among the better established in medicine.
HDL carries cholesterol back to the liver. Higher values correlate with lower risk, though attempts to raise it with drugs have not reduced events, which suggests the relationship is more complicated than the label implies.
Triglycerides are a different fat, related to diet, alcohol and metabolic health.
Why one number is not enough
Risk depends on the combination, plus age, blood pressure, smoking, diabetes and family history. A given LDL value means something different in a healthy forty-year-old than in a sixty-year-old smoker with diabetes.
This is why doctors use risk calculators rather than treating a single threshold.
What affects the numbers
Diet, particularly saturated fat intake, though individual response varies considerably. Weight, physical activity, and alcohol.
And genetics, substantially. Some people have very high cholesterol despite excellent habits, from an inherited condition that is underdiagnosed.
The honest note
This explains what the test reports. Interpretation and any decision about treatment belongs with a doctor who has your full picture.