There are three possible explanations:

• You did indeed pay for the test, but the pathologist considered it necessary to perform additional investigations on your sample (for example, an immunohistochemical [IHC] study) to provide further details for the report, which resulted in another bill.
• Your doctor prescribed a three-day cytology test. This involves three different examinations, resulting in three separate bills.
• You sent your payment after an initial reminder was issued. In other words, our correspondence crossed paths. In this case you do not need to take any action: our billing department will adjust your account shortly.