Background: Add-on test requests, where a clinician requests further test assays on an existing specimen, contribute disproportionately to pathology service workload. However, little research has quantified the volume, rates, source or types of add-on tests. This study provides a descriptive analysis of add-on testing within a pathology service serving five hospitals. Methods: We analyzed 6 months of test data extracted from a pathology service in metropolitan Sydney, Australia. Add-on requests were analyzed in terms of total volume and as a proportion of all test requests and test assays; ten most frequently requested add-on test types for clinical chemistry and hematology; by patient registration category; and proportions of add-on requests received within 1-, 4-, 8-, and 24-h of specimen collection. Results: Add-on test requests constituted 3.7 % (n = 19,541) of the total 529,361 test requests. Clinical chemistry and hematology add-on requests accounted for 76.9 % of all add-on requests. The add-on request rate was higher in the clinical chemistry (5.4 %) than in hematology (1.3 %). Patients who entered hospital via the emergency department had the highest rates of add-on requests. A total of 79.5 % of add-on requests across the pathology service were made within 24-h of specimen collection. Conclusions: The volume of add-on requests is substantial and varies considerably by test type and patient registration category thus impacting differentially upon pathology service departments. While some add-on requests are unavoidable in clinical practice, others are precipitated by inadequate information at the point of care. Improving appropriate utilization of add-on testing will reduce their burden on pathology services.