Generally ISO has been very general about trending. Thou shalt do it. In some cases, tests are indeed go-no go and there is no "measurement" made. In that case, one can perform the trending on a p-chart (percentage or proportion) control chart. An advantage of using SPC control charts in conjunction with ISO is SPC is a pretty tried and true method that an auditor cannot say is an improper choice, as long as you follow your own rules established in your own procedure.