Unlocking the Truth: Cross-Examining a Blood Test Analyst at a Forensic Laboratory

Kimberly2 1Author:

Kimberly J. Benjamin, Founder & Managing Attorney

Blood test analysis plays a crucial role in forensic investigations, helping to identify and substantiate evidence in criminal cases. The process involves the careful examination of blood samples to determine the presence of alcohol and drugs, establish their quantity, and provide reliable evidence for prosecutors or defendants. Cross-examining the blood test analyst during a trial is a crucial step in ensuring the accuracy, reliability, and credibility of these results. In this article, we will explore the key steps to effectively cross-examine a blood test analyst at a forensic laboratory.


1. Know the Scientific Concepts:

Before beginning a cross-examination, it is paramount to have a solid understanding of scientific concepts related to blood analysis. Familiarize yourself with the methodologies, technical jargon, instruments, and protocols commonly used in forensic laboratories. This knowledge will help you pinpoint any potential errors, inconsistencies, or limitations in the analyst’s testimony.


2. Prepare Thoroughly:

Preparing for the cross-examination is critical for a successful attack or defense. Obtain copies of not just the blood test report, but also all the records from the lab, such as: laboratory notes, manuals, and any relevant scientific literature for your case. Analyze the methodology and procedures used by the analyst, paying particular attention to potential sources of error, contamination, or misinterpretation.


3. Challenge Competence and Qualifications:

Start your cross-examination by effectively questioning the analyst’s qualifications and expertise in the field. Ask about their educational background, training, experience, and any certifications held to evaluate their level of competence and credibility. Uncover any potential biases or conflicts of interest that may cast doubt on their objectivity and impartiality.


4. Scrutinize the Collection and Handling of Samples:

Be sure you explore how the blood vial kit was stored prior to usage. If the kit ever was too cold or too hot, the seal (rubber O-ring) in the vial may be compromised [remember this is what caused the Challenger disaster]. A compromised seal may cause air to enter the vial, which may lead to bacteria growth and contamination of the blood, particularly fermentation. Also, probe into the details of how the blood sample was collected, transported, and subsequently handled by the laboratory. Many errors that are fodder for effective cross examination occur prior to the blood arriving at the lab due to failure to properly collect and store the blood, in particular a failure to refrigerate at the proper temperatures in a calibrated and maintained refrigerator. Establish whether proper chain of custody protocols were followed to ensure the integrity and reliability of the evidence. Inquire about any environmental factors, storage conditions, or sample contamination risks that might influence the accuracy of the testing.


5. Highlight Potential Lab Errors and Quality Control Measures:

Inquire about the laboratory’s quality control procedures and ascertain whether these were properly executed. Know their Standard Operating

Procedures (SOPs) better than they do and compare them to the standards of highly reputable members of the scientific community. Most forensic labs in the U.S. fail to follow the highest of standards established by the International Organization for Standardization (ISO), which is an independent, non-governmental organization that develops standards to ensure the quality, safety, and efficiency of products, services, and systems. Investigate the reliability and accuracy of the analytical instruments utilized, questioning the calibration, maintenance, and regularity of performance checks. Seek information regarding the proficiency testing conducted on the analyst and the laboratory’s overall track record for excellence.


6. Address Possible Interpretation and Misinterpretation Issues:

Carefully scrutinize the analyst’s interpretation and understanding of the test results. Determine if alternative explanations or hypotheses could lead to different conclusions [reasonable doubt]. Identify uncertainties, false positives, false negatives, or potential misinterpretations that may compromise the validity of the results presented. False results are more common in a forensic lab setting than the general public, and even defense lawyers, ever imagined. On CSI, things appear to be perfect and trustworthy. The same is not true in real life.


7. Seek Clarification on Limitations and Assumptions:

Press the analyst to clarify any limitations and assumptions made during the testing process. Probe into the accuracy and precision of the machines used, any type of interferences, cross-reactivity concerns, or inherent limitations of the techniques employed. Understanding these limitations will

help expose potential flaws in the analysis and cast doubt on the reliability of the results.



Cross-examination plays a crucial role in the judicial process, particularly when challenging the conclusions drawn from blood test analysis in forensic laboratory settings. By thoroughly understanding the scientific principles, preparing diligently, and asking pertinent questions, legal representatives can effectively challenge the testimony of a blood test analyst. By highlighting potential errors, limitations, and interpretations, a balanced and fair examination can help ensure justice is served.

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