The   diagnosis   of   VZV   infection   is   usually   clinical.   If   the cutaneous presentation is atypical, or laboratory confirmation is required, VZV polymerase chain reaction (PCR) is used. In urgent cases, immunofluorescence can be performed within 2 hours, or a Tzanck smear considered. PCR for viral nucleic acid is highly sensitive and can also distinguish between HSV and VZV.[13] PCR is typically performed on vesicular fluid.[14] Antibody testing for VZV immunoglobulin IgM and IgG detects either acute infection or past exposure. Measurement of VZV IgG can determine immunity to varicella, which is useful in determining if patients need zoster immunoglobulin after varicella exposure, or varicella vaccination. VZV IgM is detected in acute chickenpox and in about 70% of people with HZ.[15]