The prothrombin time (PT) is done either to evaluate a bleeding disorder or monitor the effect of ‘blood thinners.’ As the test result is dependent on the reagents and methodology used and varies from lab to lab, the WHO encouraged it to be expressed in terms of a ratio called INR so that the test results are standardized.
You have not mentioned why this test is being done but I presume it is to monitor the intake of oral anticoagulants. These drugs are given to patients who have suffered a heart attack / stroke / clotting within a blood vessel / undergone certain type of surgeries etc. The dose is titrated depending on the INR so that it may not be excessive and lead to bleeding. Depending on the underlying disorder, the INR may be kept between 2.0 to 3.5.
Many drugs (antibiotics, pain killers, antacids, oral pill, anti-epilepsy drugs, multivitamin preparations, diuretics, anti-allergy medications, etc) and dietary substances like green leafy vegetables, liver, soya bean etc. can interfere with your drug and affect the INR. Cholesterol does not affect it unless the blood levels are so high that the specimen becomes lipemic (milky white) and testing becomes difficult.
You need to discuss with your doctor all the medicines that you are taking and adhere to a schedule of drug intake.