Health Security Cess audit requires notice, document verification, opportunity to reply, and finalisation of findings. Rule 18 sets the audit period as a financial year or part/multiples thereof, requires at least fifteen days' notice before audit, authorises the proper officer and an assisting team to verify books, records, returns and declarations and record observations in audit notes, allows the officer to inform the registered person of discrepancies, permits the person to file a reply, and requires the officer to finalise audit findings after considering that reply.