A sample analyzer includes a sample preparation device, an impedance testing device, an optical testing device, and a controller. The controller is configured to control the impedance testing device to test a first test sample to obtain a first platelet counting result for a test blood sample based on electronic information of the first test sample; control the optical testing device to test a second test sample to obtain a second platelet counting result for the test blood sample based only on optical information of the second test sample or based on both the electronic information of the first test sample and the optical information of the second test sample; determine whether the first platelet counting result is unreliable due to abnormality of the test blood sample; and output the first and/or the second platelet counting result according to the result of the determination.