Diagnostic Accuracy of 2 cm Versus 4 cm Insertion Depth for Nasal Swabs for SARS-CoV-2 Rapid Antigen Testing-A...

Background/Objectives: Optimal specimen collection is essential for accurate diagnostic tests for upper airway infections. Rapid antigen diagnostic tests (RDTs) are commonly used for SARS-CoV-2 testing, yet the optimal sampling depth remains unclear. This study aimed to compare the diagnostic sensitivity and patient discomfort associated with two nasal swab depths: 2 cm (anterior nasal) and 4 cm (proposed mid-turbinate). Methods: In this randomized, paired clinical trial conducted at a public COVID-19 test center in Copenhagen, Denmark, 309 adults presenting for SARS-CoV-2 RT-PCR testing were enrolled. Each participant underwent bilateral nasal sampling using RDTs: one nostril with a 2 cm swab and the other with a 4 cm swab, randomized by side. RT-PCR from oropharyngeal swabs served as the reference standard. Discomfort was rated using a 10-point visual analog scale (VAS). Results: Among the 309 participants, 57 (18.4%) tested positive for SARS-CoV-2 by RT-PCR. RDT sensitivity was 62.1% (95% CI: 48.4-74.5%) for 2 cm swabs and 70.2% (95% CI: 56.6-81.6%) for 4 cm swabs, a non-significant difference (p = 0.34). Among symptomatic individuals, sensitivity increased to 74.4% (2 cm) and 86.0% (4 cm), though the difference also remained non-significant (p = 0.17). Discomfort scores were significantly higher for the 4 cm swab (mean VAS: 5.2) compared to 2 cm (mean VAS: 3.8; p < 0.001). Conclusions: While not statistically significant, deeper mid-turbinate swabbing (4 cm) showed higher diagnostic sensitivity than anterior nasal swabbing (2 cm), especially in symptomatic individuals. However, this came at the cost of increased discomfort. These findings highlight the importance of balancing diagnostic performance and patient tolerability in pandemic testing strategies. The study contributes valuable evidence to inform future guideline development, particularly regarding swab technique, test accuracy, and feasibility in clinical and public health settings.