Isomer-resolved lipidomics needs a direct analytical connection between structural assignment and relative abundance. Summed-composition names can merge distinct phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol arrangements into one lipid entry even when fatty-acyl placement on glycerol carries biochemical significance. This work establishes a diagnostic-fragment-gated version of Diagnostic Reporter-Ion Normalization (DRIN) for multistage mass spectrometry, in which quantitative ratios are accepted only when reporter ions arise from an assigned sn-positional fragment. The central question is whether reporter-ion measurements can be converted into calibrated and chemically interpretable lipid-isomer maps while keeping response linearity, matrix transfer, and plasma directionality as separate analytical claims. The measurement set comprises PC 18:1/16:0 calibration values, a two-concentration Escherichia coli lipid-extract comparison, and a human plasma phosphatidylcholine isomer profile. PC 18:1/16:0 produced near-proportional reporter response at both lipid and sn-isomer levels, with slopes of 1.0906 and 1.1274. In the bacterial extract comparison, 15 assigned PG and PE isomers recovered the intended two-fold relation with 87.0–97.5% accuracy. In plasma, PC 18:0/18:2, PC 16:0/18:2, PC 18:1/16:0, and PC 18:0/18:1 showed the strongest enrichment, whereas PC 14:0/20:4, PC 16:0/20:5, and PC 18:0/22:6 showed pronounced depletion. These findings answer the analytical question by showing that reporter-ion ratios become defensible isomer ratios only when diagnostic-fragment identity, calibration behavior, and matrix compatibility are evaluated together.