This laboratory experiment evaluated the effectiveness of three presumptive blood-testing kits—phenolphthalein (KM), leucocrystal violet (LCV), and tetramethylbenzidine (TMB)—in distinguishing actual blood from substances with similar visual appearance. Six samples (red paint, potato, tomato sauce, food coloring, raw beet, and known blood) were tested using standardized protocols with positive and negative controls. Results demonstrated that all three kits correctly identified the known blood sample and ruled out most false positives, though tomato sauce and raw beet produced false-positive reactions requiring retesting. The findings highlight the importance of confirmatory testing in forensic blood analysis and the limitations of presumptive tests as standalone identification methods.
The term scientific method refers to a systematic way of investigation and acquisition of knowledge through the testing of a theory or hypothesis. This process involves working through measurements, observations, and empirical notes to produce results that prove or disprove the original theory. The basic method consists of five steps: (a) formulating a question or hypothesis, (b) designing an experiment or means of collecting data, (c) conducting observation or experimentation, (d) analyzing the results and evaluating proof or disproof of the hypothesis, and (e) suggesting directions for future research (Cary, 2003).
The purpose of this laboratory was to observe and evaluate the reactions of various presumptive blood-testing kits on suspected bloodstains in order to assess whether the stains show positive, negative, or false-positive reactions. Presumptive blood tests are screening tools used in forensic analysis to determine whether a substance warrants further investigation as potential biological evidence.
Three presumptive testing kits were employed: phenolphthalein (KM), leucocrystal violet (LCV), and tetramethylbenzidine (TMB). Swabbed samples from known bloodstain cards were used for comparison, and color changes were carefully observed. Areas adjacent to each stain were also swabbed to control for background contamination. Results from test substances were compared against known control agents.
Before conducting tests on unknown samples, a positive control test was completed using the known blood sample to verify that each kit produced expected color reactions. A negative control was similarly established to confirm the absence of false positives in the testing environment. Tetramethylbenzidine (TMB) was used as the positive control to confirm kit functionality. To minimize contamination and verify reactive results, duplicate testing was conducted on all samples. Care was taken throughout to prevent cross-contamination between samples and to maintain the integrity of control materials.
Six samples were selected for testing based on their visual similarity to blood or their potential to contain hemoglobin-like substances. The test materials included red paint, smashed potato, cooked tomato sauce, red food coloring, fresh raw beet, and actual blood from a control test kit. These substances were chosen because they could outwardly appear to be blood and, by their color, might suggest the presence of hemoglobin.
The results for all presumptive tests are summarized in Table 1. The known blood sample produced the expected color reactions across all three kits: pink with phenolphthalein, purple with leucocrystal violet, and green with tetramethylbenzidine, confirming the positive control and kit functionality.
Among the non-blood substances, red paint, potato, and red food coloring all tested negative (N) across all three kits, correctly ruling out these materials as blood. However, two samples produced false-positive (FP) reactions: cooked tomato sauce showed positive results with all three kits, and fresh raw beet similarly tested positive with KM, LCV, and TMB. These false positives required retesting and additional evaluation to reach accurate conclusions. The false-positive results from tomato sauce and raw beet highlight the importance of using multiple reagents and confirmatory procedures when presumptive tests produce ambiguous outcomes.
The experimental results demonstrate that presumptive blood-testing kits are effective screening tools for identifying blood at crime scenes, yet they also reveal the necessity of confirmatory testing when anomalous reactions occur. The collection and preservation of blood evidence from crime scenes requires rigorous chemical analysis to distinguish genuine biological material from environmental substances that may produce false positives (Schiro, 2011).
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