Influence of medical procedures and medicines on the clinical laboratory results (chemical and pharmacological interferences). Instructions for control of medicinal effects on the clinical laboratory investigations.
The interference of drugs with laboratory results can be broken down in 4 mechanisms; physical, chemical, pharmacological and drug-drug interaction.
Physical; it causes interference with the standard colorimetric, photometric and fluorometric methods in that the certain drug would impart a characteristic color to the specimen. It is also wise, in such cases of colorimetric changes to think of alterations due to drugs first.
Chemical; drugs may be measured as analytes, either because of their similarity in chemical structure or because they contain a component that is the analyte. Most severe and long-lasting of such effects is seen in the intravenous administration of drugs. A substance being determined may reacts in vivo with the drug administered. Such as in EDTA in heavy metal intoxication.
Pharmacological; interference due to the pharmacological or toxic activity of the drug. Sodium warfarin increases prothrombin time. Thiazide diuretics or glucocorticosteroids cause hypopotassemia and hyperglycemia.
Drug-Drug; drugs which independently have only little effects on the lab test, but may produce alterations upon interaction with other drugs. Colifibrate + warfarin ® increased prothrombin time. Medicines thus alter lab results. Their administration should be noted on the sheet for the lab request. For example thyroid supplement therapy or antihypertensives that alter the values. In many of these cases the patient will be advised to stop taking of the medication at the morning of blood collection to have the representation of a true result.
The investigation for example for H. pylori may produce a false negative result in a fecal antigen test in patients currently taking certain antibiotics or PPIs as these would decrease the gastric load of H. pylori. Medicines may also cause a biological effect to the patient. E.g. some antibiotics, Cardiovascular medicines, NSAIDs, GI meds and more may account for and ↑ INR in a patient taking warfarin who normally has stable INR values. The long term use of metformin or PPIs is a possible explanation of ↓ Vit. B12. Many medicines have an effect on the balance of Na and K such as diuretics that can cause both their respective elevation or lowering of values. Chemotherapy is known to decrease the WBC count and steroids have a positive effect on blood glucose levels.