Diagnostic Validation of Nalidixic Acid Disk Diffusion as a Mandatory Safeguard Against Deceptive Ciprofloxacin Susceptibility Reporting

Background: Standard disk testing often fails to detect early mutations that cause reduced fluoroquinolone susceptibility in Salmonella, leading to inadvertent treatment failures. Objective: The purpose of the present study was to validate the accuracy of the 30μg nalidixic acid disk as a low-cost screening marker for elevated ciprofloxacin MICs. Methodology: This cross-sectional study was conducted in the Department of Microbiology & Immunology at Bangladesh Medical University (BMU), Dhaka, Bangladesh, from January 2008 to December 2008 for a period of one year. Disk diffusion parameters for nalidixic acid were cross-referenced against quantitative ciprofloxacin agar dilution MICs across 100 clinical Salmonella isolates (72 Salmonella Typhi and 28 Salmonella Paratyphi A). Results: Nalidixic acid resistance (zone less than or equal to 13 mm) was found in 76.0% of isolates. This profile correlated perfectly with elevated ciprofloxacin MIC values more than or equal to 1 μg /ml (covering 40% resistant and 36% intermediately sensitive strains). The 24.0% of isolates sensitive to nalidixic acid matched true baseline ciprofloxacin-sensitive endpoints. Conclusion: Testing with a nalidixic acid disk provides high diagnostic sensitivity and specificity, making it an essential, cost-effective screening step to prevent false reporting of ciprofloxacin in resource-limited laboratories. Bangladesh Journal of Infectious Diseases, June 2026;13(1):14-18