Figure 5. An in vitro comparison of malignant hyperthermia susceptible and nonsusceptible muscle bundles (intact fiber bundles from the rectus abdominis muscle) to graded doses of either caffeine or 4-chloro-m-cresol. Contractures of greater than or equal to 200 mg of force were considered a positive response. The graded caffeine contracture test provided a 4- to 16-fold concentration difference between susceptible and normal muscle bundles, whereas the 4-chloro-m-cresol contracture responses provided a 32-fold diagnostic concentration difference.