Analytical interference in laboratory assays is not only unpredictable but also an underestimated problem. Not recognising these interferences can lead to misdiagnosis and mismanagement of patients. We present a case of a patient with chest pain and ischaemic risk factors with incongruent biochemical results. These results were discovered to be due to the presence of macro-creatine kinase (macro-CK) in vivo interfering with the CKMB activity assay.