Purpose The purpose of this study was to sonographically evaluate whether intravenous (IV) flucloxacillin administration was associated with an increased risk of peripheral intravenous catheter (PIVC) thrombus formation.

Methods This observational study included participants enrolled as a convenience sample from a larger prospective study of patients with cellulitis receiving IV antibiotics in the emergency department. Point-of-care ultrasound was used to evaluate the PIVCs for thrombus formation after insertion and at specified timepoints after IV administration of antibiotic or saline solution through to discharge. The primary endpoint included the presence and length of the thrombus in proximity of the catheter tip.

Results Between May 2021 and June 2022, 25 participants were enrolled and received either IV flucloxacillin (n = 10), other IV antibiotics (n = 8) or no IV antibiotics (control; n = 7). PIVC thrombus formation was sonographically detected in 100%, 67% and 17% of patients in flucloxacillin, other and control groups at 6–12 h (flucloxacillin vs. control; P = 0.015), with a mean length of 17.4 ± 8.1 (flucloxacillin vs. control; P = 0.46), 15.5 ± 13.4 (other vs. control; P = 0.73) and 7.3 ± 17.9 mm (control), respectively. Thrombus formation increased significantly in the flucloxacillin group over time (0–>12 h; P = 0.03) but did not increase in the other or control groups.

Discussion The administration of IV flucloxacillin appears to promote the formation of a PIVC thrombus visible on ultrasound, but the clinical implications are uncertain. Although the vast majority appear to be asymptomatic, they have the potential to become a precursor to thrombophlebitis and lead to early PIVC failure.

Conclusions It was feasible to identify and measure PIVC thrombus sonographically. Ultrasound showed that IV flucloxacillin administration appeared to be associated with more frequent formation of PIVC thrombus, with these increasing in length over time. Further research is required to confirm these findings in larger studies and to identify any clinical implications of the findings.