Acute myocardial infarction is the most common cause of morbidity and mortality in the world. Cardiac troponin measurements play a key role in the diagnosis. However, preanalytical errors as a result of the presence of fibrin or interference due to conditions such as heterophile antibody positivity may cause erroneously high results. Such errors may result in invasive procedures, such as angiography, which may add unnecessary risk. In our hospital, high-sensitivity troponin-I (hs-TnI) was routinely analyzed using a serum separator tube (reference value: female <15.5 pg/mL, male <34.2 pg/mL). This report describes the cases of 2 patients with a false initial troponin measurement: a 19-year-old male patient and a 55-year-old female patient. The hs-TnI value of the male patient was initially measured as 55.5 pg/mL. After the analysis, it was noted that the sample contained fibrin. The sample was centrifuged again and the TnI result was 1.8 pg/mL. Similarly, the TnI result of the female patient was first measured as 90.2 pg/mL. When it was observed that there was fibrin present, the sample was recentrifuged. The revised result was 2.4 pg/mL. The laboratory staff were trained on preanalytical errors, and the use of lithium heparin tubes was implemented in the laboratory as an additional means to eliminate the problem of fibrin interference.Keywords: Fibrin clot, immunoassay, interference, troponin I.