Routine post-operative bloods following all elective arthroplasty may be unnecessary. This retrospective cohort study aims to define the proportion of post-operative tests altering clinical management. Clinical coding identified all elective hip or knee joint replacement under Hawkes Bay District Health Board contract between September 2019-December 2020 (N=373). Uni-compartmental and bilateral replacements, procedures performed for cancer, and those with insufficient data were excluded. Demographics, perioperative technique, and medical complication data was collected. Pre- and post-operative blood tests were assessed. Outcome measures included clinical intervention for abnormal post-operative sodium (Na), creatinine (Cr), haemoglobin (Hb), or potassium (K) levels. A cost-benefit analysis assessed unnecessary testing. 350 patients were Included. Median age was 71 (range 34-92), with 46.9% male. Only 26 abnormal post-operative results required intervention (7.1%). 11 interventions were for low Na, 4 for low K, and 4 for elevated Cr. Only 7 patients were transfused blood products. Older age (p=0.009) and higher ASA (p=0.02) were associated with intervention of any kind. Abnormal preoperative results significantly predicted intervention for Na (p<0.05) and Cr (p<0.05). All patients requiring treatment for K used diuretic medication. Preoperative Hb level was not associated with need for transfusion. Overall, there were 1027 unnecessary investigations resulting in $18,307 excess expenditure. Our study identified that the majority of elective arthroplasty patients do not require routine