What is the primary diagnostic test used for herpes simplex virus infection?

The primary diagnostic test for herpes simplex virus (HSV) infection is polymerase chain reaction (PCR). PCR is highly sensitive and specific for detecting viral DNA, making it the preferred method for diagnosing HSV infections, particularly in cases such as encephalitis or when lesions are present that are not easily biopsied.

Culture analysis can also be used to isolate the virus; however, it is less sensitive than PCR, especially in cases where lesions have already begun healing or when there’s a low viral load. The Tzanck smear, although a historical method for diagnosing herpes infections by identifying multinucleated giant cells, lacks specificity and sensitivity when compared to molecular techniques, thus rendering it less effective in modern practice. Blood serology tests can detect antibodies to HSV but are not primary diagnostic tests for active infections, as they do not identify the presence of the virus during an active outbreak.

Therefore, while the Tzanck smear can suggest an HSV infection, the best approach according to current medical standards is PCR for its accuracy and reliability in diagnosing herpes simplex virus infections.