Background. Differential diagnosis of Low-back pain (LBP) is complex and a prominent health care issue at all Health-care levels; guidance may come from patients’ history cues and clinical examination signs. Human and animal studies report that lumbar radicular pain (LRP) may be diagnosed, at all care settings, by the evaluation of subjective responses of injured lumbar nerves to a strain applied at the buttock. The Buttock Applied Strain (BUAS-test) may guide the differential diagnosis of LBP. Following an ex-adiuvantibus criterion, clinical improvement of LRP, diagnosed with the BUAS-test and congruently treated, may support this test diagnostic ability. Methods. Among 258 LRP patients, positive at V1, to the BUAS-test (with/without positive Straight-Leg-Raising-Test, SLRT), the effect of gabapentin on painDETECT (PD) questionnaire and BPI outcomes was quantified in the follow-up visit (V2). We hypothesized that, at V2, >50% of the sample would present negative PD-outcome, significant (t-Test), and 2 points V2-V1 differences for each the BPI-item’s score. Multinomial-Logistic-Regression (MLR) and χ2 analyses were used to evaluate the PD-V2-outcomes’ dependence upon independent variables. Results. Of the sample, 77% reported Negative PD-V2-outcome. V2-V1 differences of all BPI-items were significant and >2 points. PD-V2-outcomes showed significant associations with SLRT-V1 and PD-V1, respectively, but not with gender, age group, or pain-site. MLR showed a significant relationship between SLRT-V1 and PD-V2 outcomes. Conclusions. Among LRP patients, diagnosed by the BUAS-test and treated with gabapentin, all prespecified endpoints were reached. These results may be considered a piece of ex-adiuvantibus evidence for the BUAS-test ability to diagnose LRP. While positive BUAS-test implies potential LRP, the co-presence with positive SLRT may imply a severer LRP condition. Further prospective research, in different settings and direct clinical measures, is needed.