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Immunoassay procedures, such as enzyme-linked immunosorbent assay (ELISA), are widely used for screening samples in both driving under the influence of drugs (DUID) and postmortem (PM) investigations. While these are sensitive and widely used techniques, they lack specificity compared to more novel instrumental screening platforms. In this study, the cross-reactivities of several cannabinoid isomers and related compounds were evaluated in whole blood using the Cannabinoids Direct ELISA kit from Immunalysis. The compounds of interest were supplemented individually at three different concentrations, ranging from 10 to 100 ng/mL or 10 to 1,000 ng/mL depending on analyte, to determine initial feasibility. Compounds exhibiting cross-reactivity were then tested to create dose–response curves to calculate the percent cross-reactivity. The cross-reactivity was determined to be 200% for delta-8-carboxy-tetrahydrocannabinol (THC) (delta-8-carboxy-THC), 25% for delta-9,11-THC, 13% for delta-10-THC, 7% for delta-6a(10a)-THC, 3% for THC-O-acetate and 0.5% for tetrahydrocannabiphorol. To determine potential impacts on forensic laboratory casework, a review of DUID and PM casework was also performed. From November 2020 to June 2021, a random sampling of DUID and PM cases was selected monthly and evaluated for the presence of cannabinoid isomer(s) in the absence of a reportable delta-9-carboxy-THC result. While validated techniques for the identification and confirmation of these isomer(s) did not exist at the time of routine testing, delta-8-carboxy-THC was believed to be the most common isomer finding based on current testing capability. This study demonstrated a noticeable increase in the presence of isomeric cannabinoid compounds in both forensic DUID and PM casework sampled during this period and suggests potential impacts for clinical casework as well.