Chapter 7: Laboratory Principles

INTRODUCTION

Clinical toxicology addresses the harm caused by acute and chronic exposures to excessive amounts of a xenobiotic. Detecting the presence or measuring the concentration of toxic xenobiotics is the primary activity of the analytical toxicology laboratory. Such testing is closely intertwined with therapeutic drug monitoring (TDM), in which drug concentrations are measured as an aid to optimizing drug dosing regimens. Another offshoot of laboratory toxicology that has become increasingly requested is testing for drugs of abuse.

The goal of the hospital toxicology laboratory is to provide clinically relevant test results to support the management of poisoned or intoxicated patients.

To be of optimal clinical value, the laboratory service requires both an appropriate test menu that meets quality criteria, as well as the ability to provide results within a clinically relevant time frame.

RECOMMENDATIONS FOR ROUTINELY AVAILABLE TOXICOLOGY TESTS

Despite a common focus, there is remarkable variability in the range of tests offered by clinical toxicologic laboratories. Test menus range from batched testing for routinely monitored drugs and common drugs of abuse to around-the-clock availability of a broad arsenal of assays with the potential to identify thousands of compounds. For most laboratories, it would be entirely impractical and inefficient to attempt to provide a full range of analyses in real-time because of cost constraints, staffing issues, and the required technical expertise.

Decisions on the test menu and turn-around times (TATs) to be offered should be made by the laboratory director in consultation with the medical toxicologists and other clinicians who will use the service. They should take into account regional patterns of the use of both licit and illicit xenobiotics and exposure to environmental xenobiotics as well as the available resources and competing priorities.

The essential questions that need to be addressed include:

1. Which tests should be available?

2. Do the assays require qualitative or quantitative results?

3. What specimen matrices will be tested (eg, blood, urine, other body fluid)?

4. When and how should the specimen be obtained?

5. What is the acceptable TAT?

From a consensus process that involved clinical biochemists, medical toxicologists, forensic toxicologists, and emergency physicians, the National Academy of Clinical Biochemists (NACB) recommended that hospital laboratories provide a two-tiered approach to toxicology testing.²⁷ In the United Kingdom, the National Poisons Information Service and the Association of Clinical Biochemists recommend a nearly identical list of tests, omitting the anticonvulsants.²¹

Tier 1 (Table 7–1) includes basic tests that should be offered locally because of their clinical relevance and the technical feasibility of rapid TATs. As with all laboratory tests, they should be selectively ordered based on the history, clinical presentation, and other relevant factors and not simply ordered as a general screening panel for all suspected cases of poisoning. The recommended TATs for most of the tests listed was 1 ...