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4.20: Toxicology

  • Page ID
    122361
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    The Toxicology Laboratory is asked to measure acetaminophen on a stat basis for a suspected overdose. The result of the analysis is reported back as 200 g/mL. The Emergency Room resident calls the laboratory for interpretation of the result, i.e., does the result indicate toxicity or not? The medical technologist asks the resident how soon after the suspected overdose the specimen was drawn. The resident, unable to obtain a history, does not know. The technologist asks the resident to obtain a second specimen in four hours to evaluate the patient’s status.

    QUESTIONS

    1. Why was a second specimen requested?
    2. The result of the second blood analysis for acetaminophen is 150 \(\mu\)g/mL. Does this second value help the laboratory to evaluate the toxicity of the ingested acetaminophen?

    Questions to Consider

    1. Is there a relationship between blood levels of acetaminophen and toxicity?
    2. Why can’t the laboratory advise the physician about the toxicity of the 200 \(\mu\)g/mL acetaminophen value?
    3. Are there other reasons why a second specimen, several hours after the first one, might be important in determining the care of the patient?
    Answer
    1. By determining whether the acetominophen level of the second specimen is higher or lower than that of the initial one, the Toxicology Laboratory will enable the physician to decide whether the drug is in the absorption or elimination phase.
    2. Although the second value is lower than the first, it is still quite high, suggesting that the patient is in the early part of the elimination phase. With these blood levels of acetaminophen, the physician would probably prescribe an antidote and treat the patient. The antidote for acetaminophen poisoning is N-acetylcystamine (See Table 51-7, p 1000).

    Answers to Questions to Consider

    1. There is a definite relationship between blood levels of acetaminophen and toxicity as shown in the in the Acetaminophen Chapter on the Methods CD-ROM and Figure 51-6 of the 4th Edition (p 1007).
    2. As Fig. 51-6 demonstrates, the toxicity associated with a specific acetaminophen blood level is very dependent upon the time after drug ingestion. A 200 \(\mu\)g/mL level six hours after ingestion will be associated with mild to severe liver damage. However, that same level at 12 hours is considered extremely toxic, while at 3 hours after overdose the level would be considered clinically insignificant. Thus a single blood level without a good history is of limited use unless the level of acetaminophen is very high (i.e. greater than 250 \(\mu\)g/mL), or very low, i.e less than 100 \(\mu\)g/mL.
    3. Two specimens drawn several hours apart can be used to determine whether the patient is still absorbing the drug or is in the elimination stage (Fig. 51-6). The patient is at greater risk for liver damage if the sample is drawn in the absorption phase, since the blood levels will increase. In that situation, usually more aggressive intervention measures will be taken. If the patient is in the elimination phase, the risk for additional toxicity is reduced.

    This page titled 4.20: Toxicology is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Lawrence Kaplan & Amadeo Pesce.

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