Chemistry, toxicology & urinalysis

• Multidisciplinary field focusing on the diagnosis, management and prevention of chemical agent toxicities in living systems, including
  • Acute or chronic poisoning, drug overdoses, employment drug screening, industrial accidents, environmental exposures, envenomations, adverse drug reactions and therapeutic drug monitoring
• Due to a wide range of potential chemical entities faced in practice, several analytical techniques may be required to detect the toxin of interest

• Most drugs are excreted in urine (J Nucl Med Technol 2018;46:221)
  • Either unaffected or converted to metabolites of the parent drug
  • Some drugs enter enterohepatic circulation and are excreted in stool
• Most metabolism occurs in liver (cytochrome P450 system) (Hepat Mon 2016;16:e32636)
• Useful parameters include bioavailability, half life, therapeutic range, toxic levels, transport, metabolism, elimination, steady state and mechanism of action

Drug elimination kinetics

• Zero order (linear): constant quantity of drug is eliminated per unit time (J Nucl Med Technol 2018;46:221)
  • Examples: phenytoin, ethanol, warfarin, heparin, salicylates and theophylline
• First order (nonlinear): constant fraction of drug is eliminated per unit time (J Nucl Med Technol 2018;46:221)
  • Most (> 95%) drugs used at therapeutic levels are eliminated this way
• See Diagrams / tables, figure 3

Drug half life

• Time taken for half of the drug that was initially present in serum to be excreted
• Many drugs have a half life independent of their concentrations because they are eliminated via first order kinetics

Steady state

• Represents overall absorption of drug that is in dynamic equilibrium with its elimination (J Nucl Med Technol 2018;46:81)
• Reached after about 4 - 5 half lives with continuous drug administration

Bioavailability

• Fraction of unchanged drug reaching systemic circulation (J Nucl Med Technol 2018;46:221)
  • Fraction of drug binds albumin in blood; remaining drug is free
  • Some small molecules compete for albumin binding, affecting total drug concentration
• Affected by physical drug properties, drug formulations, route of administration, fed versus fasted state, drug-drug interactions, plasma protein binding, disease state of individual, P450 enzyme induction / inhibition and metabolic differences

Volume of distribution (Vd)

• Total amount of drug in body/drug concentration in plasma (J Nucl Med Technol 2018;46:221)
• A theoretical volume; uniform distribution of drug between plasma and rest of body
• Lipophilic drugs have high volume of distribution (molecules sequestered in adipose tissue); hydrophilic drugs have low volume of distribution

Lethal dose (LD50)

• Amount of an ingested substance that kills 50% of test subjects; expressed in mg/kg (Regul Toxicol Pharmacol 2016;80:274)

General considerations

• Anion gap, osmolal gap and oxygen saturation gap should be determined when an overdose is suspected
• Anion gap (mmol/L) = [Na+] - [Cl- + HCO₃-] (J Emerg Nurs 2005;31:109)
  • Assessment for metabolic acidosis
    • Normal value: 8 - 16 mmol/L; metabolic acidosis likely with values > 30 mmol/L
  • Increased anion gap in methanol, acetaminophen, carbon monoxide, epinephrine, nitroprusside, formaldehyde, ethylene glycol, salicylates and propylene glycol (Clin Toxicol (Phila) 2015;53:589)
  • Lowered anion gap in hypoalbuminemia
• Osmolal gap (mOsm/kg) = (2 x Na) + (BUN/2.8) + (glucose/18) (Postgrad Med 2017;129:456)
  • Difference between serum osmolarity and calculated osmolality
  • Increased in acute alcohol poisoning (ethanol, methanol, etc.)
• Oxygen saturation gap (J Anaesthesiol Clin Pharmacol 2014;30:86)
  • Difference between oxygen saturation given by pulse oximetry and arterial blood gas
  • > 5% suggests hemoglobin-oxygen abnormality
  • Increased in agents affecting hemoglobin binding
    • Carbon monoxide, cyanide, hydrogen sulfide, methemoglobin

Contributed by Derek B. Laskar, M.D.

A 32 year old man was found dead at home. The medical examiner was called to the decedent's home to investigate the cause of death. At the decedent's home, there were multiple empty prescription pill bottles scattered around and an in situ needle was found within the vein of the man's antecubital fossa. Foaming around the nose and mouth was also noted. Which of the following techniques should be used to definitively identify all the potential opiates the deceased may have ingested?

1. Enzyme mediated immunologic technique
2. Fluorescence polarization immunoassay
3. Gas chromatography / mass spectrometry
4. High pressure liquid chromatography / mass spectrometry
5. Ion absorption spectroscopy

D. High pressure liquid chromatography should be used to definitely characterize all the opiates ingested by the decedent. Enzyme mediated immunologic technique and fluorescence polarization immunoassay are immunochemical assays used as screening tests but are not definitive. Gas chromatography / mass spectrometry is possible but sample preparation (i.e. derivatization of the compounds) would add additional steps and make the sample preparation more labor intensive and time consuming. Gas chromatography / mass spectrometry is used for mostly nonpolar analytes of interest.

Comment Here

Reference: Toxicology-general

Which of the following drugs is eliminated from the body following a nonlinear (exponential) kinetic elimination process?

1. Aspirin
2. Ethanol
3. Heparin
4. Phenytoin
5. Propranolol
6. Warfarin

E. Propranolol (and > 95% of available drugs) follow first order elimination kinetics (i.e. constant fraction per unit time). The remaining < 5% of drugs follow zero order elimination kinetics (i.e. constant amount per unit time).

Comment Here

Reference: Toxicology-general