Reliability and Concurrent Validity of 5cm-Microplastic Meter Ruler and 5mm Diameter-Nozzle Syringe for Dosage specification using Topical Diclofenac Gel and Methyl Salicilate Cream formulations

Abstract Background: Finger Tip Unit (FTU) is used for dosage specifications of topical medications but has no metric value, hence, there is risk of over-dosage. Portable tools are being proposed but there is dearth of empirical data to support their reliability. The study aimed to determine the reliability and concurrent-validity of a 5cm-Microplastic ruler and a 5mm-diameter nozzle syringe. Methods: 621 patients with musculoskeletal disorders participated in the correlation study. The test instruments were 5cm-Microplastic ruler, 5mm-diameter nozzle syringe, methyl salicylate cream and diclofenac gel. Each participant (Tester 1) measured the Finger Tip Unit Metric Length (FTUML) and the investigators (Tester 2) re-measured. Both testers ejected the cream from a standard tube on a 5cm-Microplastic ruler to reproduce the initial FTUML, and quantified the length of cream ejected. Both testers were blinded to each other’s measurements. The same procedures were adopted for the gel. The modified syringe with simulated 5mm opening was filled to 6mL with the cream or gel, and both Testers ejected the medication to produce one FTU. The difference between 6mL and the remaining volume was recorded as the quantity of cream or gel ejected. Measurements were documented on two testing days, with test-retest interval of 4 weeks. Data were analyzed using descriptive statistics and Pearson’s product-moment correlation co-efficient. Results: Both within and across the testers, significant correlations were obtained for the quantification the length of the cream and gel using the 5cm-Microplastic ruler, ranged from r = 0.23 to 0.88 (p = 0.001). Using the syringe, significant correlations were also observed for the cream and gel (r = 0.41 to 0.91, p = 0.001). There were low negative correlations with the amount cream and gel formulations ejected for both tools (r = -0.10, p = 0.01; r = -0.09, p = 0.02). Conclusion: Moderate intra- and inter-tester agreement was observed for cream and gel dosages when measured with the 5cm-Microplastic ruler and 5mm-diameter syringe over a 4-week test-retest interval. This indicates potential reliability, noting that correlation coefficients alone do not confirm reliability. However, there was no strong evidence of agreement between the two measurement tools.