Result Interpretation

Interpreting toxicology test results is multifaceted. This section describes the steps to follow in order to properly interpret

General Principles of Test Result Interpretation

Quantitative vs. Qualitative Results
- Qualitative: Indicates the presence or absence of a substance (e.g., positive or negative)
- Quantitative: Provides a numerical value for the concentration of a substance
Cutoff Values
- The concentration above which a test is considered positive
- Cutoff values are established based on analytical sensitivity and clinical relevance
Units of Measurement
- Be familiar with the units of measurement used by the laboratory (e.g., ng/mL, μg/mL, mg/L)
Reference Intervals
- Toxicology tests typically do not have “normal” reference intervals, as the goal is to detect the presence of toxic substances
- However, therapeutic ranges may be available for some drugs
Chain of Custody
- Verify the chain of custody to ensure the integrity and validity of the specimen
Positive and Negative Controls
- Ensure that positive and negative controls are within acceptable limits
Interfering Substances
- Be aware of potential interfering substances that can affect test results

Patient History
- Age, sex, weight, medical conditions, medications, and substance use history
Time of Sample Collection
- Time since ingestion, inhalation, or exposure
Route of Exposure
- Ingestion, inhalation, dermal, injection
Metabolism and Elimination
- Liver and kidney function
- Genetic factors affecting drug metabolism
Drug Interactions
- Concurrent use of other drugs
Analytical Limitations
- Cross-reactivity, sensitivity, and specificity of the assay

Verify Patient Information: Confirm the patient’s identity and demographic information
Review Analytical Data * Check QC results and calibration data * Assess the validity of the results
Assess Qualitative Results * If positive, proceed to confirmation and quantitative analysis * If negative, consider the possibility of a false-negative result due to low concentration, interfering substances, or limitations of the assay
Evaluate Quantitative Results * Compare the concentration to cutoff values and therapeutic ranges * Consider the dose-response relationship
Correlate with Clinical Information * Interpret the results in the context of the patient’s clinical presentation, medical history, and other laboratory data
Consider Interfering Factors * Assess potential drug interactions and interfering substances
Provide an Interpretation * Summarize the findings and provide a concise interpretation of the results * Explain the clinical significance of the results * Offer recommendations for further testing or management

Typical Analytes: Ethanol, methanol, isopropanol, ethylene glycol
Reporting Units: mg/dL, g/dL
Cutoff Values: Vary depending on the substance and the legal or clinical context
Factors to Consider
- Ethanol: Legal limits for driving under the influence (DUI)
- Methanol and ethylene glycol: Toxic metabolites can cause severe acidosis and organ damage
- Osmolal Gap: The difference between measured and calculated osmolality can suggest the presence of unmeasured alcohols
Clinical Significance: Intoxication, poisoning, differential diagnosis of altered mental status

Typical Analytes: Lead, mercury, arsenic, cadmium
Reporting Units: μg/dL, μg/L
Cutoff Values: Vary depending on the metal and the regulatory guidelines
Factors to Consider
- Lead: Elevated levels can cause neurological damage, developmental delays, and anemia
- Mercury: Different forms of mercury (elemental, inorganic, organic) have different toxicities
- Arsenic: Acute and chronic exposure can cause various health effects, including cancer
Clinical Significance: Environmental or occupational exposure, poisoning, chronic toxicity

Typical Analytes: Acetaminophen, salicylates, opioids, NSAIDs
Reporting Units: μg/mL, mg/L
Cutoff Values: Vary depending on the drug and the clinical context
Factors to Consider
- Acetaminophen: Risk of hepatotoxicity
- Salicylates: Risk of metabolic acidosis and respiratory alkalosis
- Opioids: Respiratory depression, altered mental status
Clinical Significance: Overdose, toxicity, monitoring of therapeutic drug levels

Typical Analytes: Amphetamines, cocaine, opiates, cannabinoids, benzodiazepines, barbiturates, etc
Reporting Units: ng/mL, μg/mL
Cutoff Values: Vary depending on the drug and the testing purpose (e.g., forensic, workplace, clinical)
Factors to Consider
- Metabolic pathways: Different metabolites may be detected depending on the assay
- Cross-reactivity: Some assays may cross-react with structurally similar compounds
Clinical Significance: Drug abuse, overdose, monitoring of drug treatment programs

Typical Analytes: Digoxin
Reporting Units: ng/mL
Therapeutic Range: 0.5 - 2.0 ng/mL (Varies based on condition/patient)
Factors to Consider
- Clinical picture: Underlying cardio condition and additional meds
- Electrolyte levels: Potassium, Magnesium
Clinical Significance: to verify and avoid toxic levels that cause side effects

Typical Analytes: Phenobarbital
Reporting Units: μg/mL
Therapeutic Range: 15 - 40 μg/mL (Varies based on patient)
Factors to Consider
- The patient’s additional meds
- The patient’s liver/kidney function
Clinical Significance: To maintain safe and therapeutic levels

Cut-off Values: They are set for specific tests
Drugs not included: All drug panels do not test for every single drug
Patients can metabolize drugs differently: The patient’s body and disease states will also affect how the drug is metabolized

Cutoff Value: The concentration above which a test is considered positive
Reference Interval: The range of values found in healthy individuals
Clinical Context: The patient’s clinical history, physical examination findings, and other laboratory data
Cross-Reactivity: The ability of an antibody to bind to multiple antigens
Metabolite: A product of drug metabolism
Drug Interaction: The effect of one drug on the ADME of another drug
Toxidrome: A group of signs and symptoms associated with a particular class of toxic substances
Forensic Toxicology: The application of toxicology to legal investigations
Workplace Drug Testing: Testing employees for drug use
Therapeutic Range: Concentration range in which effects are most commonly observed