Due professional care is exercised when audits are carried out in accordance with the standards set for the profession. The auditor is usually obligated to exercise due professional care by the terms of the engagement letter; the obligation exists even if it is not specifically stated in the engagement letter.
Proving that an auditor has exercised due professional care constitutes a complete defense against any charge brought by a plaintiff. This proof constitutes the bulk of the work in defending against lawsuits. Unfortunately for the auditor, proving one’s case can be difficult when subsequent events turn out to be worse than the auditor could have expected, in which case the defense needs to also address the issue of whether the auditor should have seen a future event coming.