Background: Lumbosacral transitional vertebra (LSTV) are congenital anomalies of spine which usually presents as either sacralisation of the L5 lumbar vertebra or lumbarisation of the S1 segment of sacrum with a prevalence of 4%-30% in world population. In plain radiographs we encounter these anomalies incidentally. Correct detection by X-Ray followed by proper numbering with MR TOP BOTTOM pasted image is essential. Wrong localisation can cause failed back syndrome. Objective: In 312 patients we foundfocal convexity at upper lateral margins of sacrum in plain X-Ray L.S (AP) view that we named “SACRAL HUMP”. The accuracy of “SACRAL HUMP” in detecting lumbo-sacral transitional vertebra in plain radiographs by comparing with MR findings using localizers (TOP-BOTTOM pasted image) was the main aim of this study. Method: Observational type of cross-sectional study was done over 312 suspected LSTV cases with low back pain sent to Department of Radio-diagnosis from PMR, Orthopaedics and other OPD for conventional/digital X-Ray L-S spine. The results of X-Ray and MRI were correlated using ROC curve analysis and inter-rater agreement using kappa value. Results: Out of 312 cases, 230 had “sacral hump” morphology. The ability of “SACRAL HUMP” morphology to detect and correctly number LSTV segment in plain radiograph showed high sensitivity (100%,100%), specificity (90%,97%), P-value <0.001 and Kappa value of 0.91,0, 92 for sacralisation and lumbarisation respectively comparable to Gold standard (MR). Conclusion: Our observation of a specific contour variation-” sacral hump” has high positive predictive value, sensitivity and specificity in detecting and numbering LSTV (comparable to gold standard) and hence can be used as a screening test in non- traumatic low back painpatients not attributing to known degenerative, infective or inflammatory causes.