Clinical biochemistry

Reference range:

OG < 10 mmol/L

The role of the laboratory in suspected poisoning by a toxic substance is irreplaceable. In the case of oral intoxication, it is necessary to take into account the time required for the absorption of the noxious substance - it appears in the blood and later in the urine. In special toxicological indications, especially in chronic poisoning) we also use less common biological materials such as hair and nails.

The most frequently encountered cases are acute ethanol poisoning and massively active substances in general. This is where the calculation of the osmolal gap (OG) - the difference between the measured and calculated osmolality-helps us. The calculated osmolality expresses the expected osmolality formed by endogenous substances and is calculated as 2 x Na + urea + glucose (see also above Determination of Na, K, Cl). Subtracting it from the instrument-determined - real osmolality of the patient gives the difference, which contains just exogenous osmotically active substances such as ethanol, methanol or ethylene glycol. The ethanol level is determined acutely by most laboratories - we can estimate the contribution of ethanol to OG by multiplying the ethanol concentration in blood (g/L) by 22. All osmotically active substances are metabolized to acids and their anions then increase the AG (see above Introduction to acid-base disturbances).

When intoxication is suspected, we very often indicate an orientation test for frequently abused substances in the urine. This is a set of parameters that usually includes the determination of amphetamines/methamphetamines, tetrahydrocannabinoids, opiates, cocaine, benzodiazepines, but also barbiturates, tricyclic antidepressants or ecstasy. The exact composition varies according to local prevalence and needs. It is important to note that this is an indicative determination based on group immunochemical reactions - the antibody used may react with various similar molecules, with their metabolites; the determination is therefore not specific. Also, the determination of the decision limit when we give a positive or negative result has no precise rules and even a negative result does not mean that the patient has not ingested the suspected nox. Add to this the uncertainty caused by the frequent efforts of patients to achieve a negative result (e.g., by ingesting large amounts of fluid and subsequent dilution of urine or various interfering additives) and the usually low correlation between the finding of positivity in the urine and clinical status. Therefore, the decision on diagnosis or therapy must be based on complex anamnestic and clinical information with support from a landmark toxicological examination.