The clinical utility and healthcare cost of antinuclear antibody testing in inpatients at a major tertiary hospital.

Antinuclear antibody (ANA) tests lack specificity and are often ordered in low pretest probability settings, leading to a low positive predictive value (PPV). Limited data exist on the clinical utility and cost of ANAs and ANA-related tests, such as extractable nuclear antigens for hospital inpatients, which this retrospective cohort study aimed to investigate. Among 183 adult inpatients who underwent ANA testing from January to February 2023 at Royal Prince Alfred Hospital, Sydney, 105 patients with a positive ANA were examined in greater detail. The primary outcomes were the PPV of ANAs in detecting autoimmune ANA-related diseases (ARDs) such as systemic lupus erythematosus, the indication for ordering the ANA, and the Medicare Benefits Schedule fees of ANAs and related tests ordered up to 6 months after the initial ANA. A total of 117 positive ANAs (including repeats) were recorded, but only two ARDs were newly diagnosed (PPV=1.7%), demonstrating a high rate of false positives. Most positive tests were ordered for workup of organ-specific dysfunction, the top three categories being hepatic (n=38), neurological (n=16), and haematological (n=13) abnormalities. The most common specific indication was undifferentiated abnormal liver function tests (LFTs) (n=27). The total fees of ANAs and related tests were AUD$11,415, equating to AUD$5,707.50 per diagnosis given a positive ANA. This study is among the first to show the low PPV of ANAs in inpatients and to quantify the high costs of ANAs and ANA-related testing in this population. This low PPV may be due to ANAs being requested contrary to the 'Choosing Wisely' guidelines which encourages more judicious ordering in low pretest probability settings such as screening in LFT abnormalities. Since the most common reason for ANA requests was for investigation of hepatic dysfunction, development of scoring tools to evaluate pretest probability before ordering ANAs should be considered.