Prediction of acetaminophen level from clinical history of overdose using a pharmacokinetic model

Delayed increase in serum acetaminophen concentration after ingestion of a combination medications: a case report

Acetaminophen is absorbed rapidly after oral intake, and serum concentration peaks within 4 hours. The Rumack–Matthew (RM) nomogram is widely used to identify the potential risk of liver dysfunction. However, the RM nomogram was intended for use only when a single agent was ingested. We report the case of a patient with overdose ingestion of an over-the-counter combination cold medication that contained acetaminophen, where the patient’s serum concentration increased over time. Over-the-counter combination cold medications are designed to relieve cold symptoms. However, the possibility that other agents that were present in the drug may change gastrointestinal kinetics should also be considered. The risk of liver dysfunction cannot be accurately determined from a single serum acetaminophen concentration measurement. Because of the risk of a delayed increase in the serum acetaminophen concentration, monitoring for liver dysfunction and developing a treatment strategy that includes N-acetylcysteine are required. This case report is targeted to clinical physicians who treat patients with acetaminophen overdose resulting from ingestion of multiple agents, and it reviews points of consideration when using the RM nomogram in acute intoxication.

Four-hour acetaminophen concentration estimation after ingested dose based on pharmacokinetic models

INTRODUCTION

The United Kingdom has recently changed the indications for N-acetylcysteine treatment for acetaminophen intoxication. Any ingestion over 75 mg/kg is now referred to the hospital. A model based on pharmacokinetic parameters was developed to predict 4-h acetaminophen concentration for this and other ingested doses.

METHODOLOGY

EMBASE and Medline were searched to obtain values for volume of distribution, absorption, and elimination constants and bioavailability for acetaminophen. Four-hour concentrations were calculated for ingestion doses currently recommended for hospital referral in different countries. Calculated plasma concentrations at 4 h for several doses were plotted against the Rumack-Matthew and the United Kingdom treatment lines.

RESULTS

Six articles were used for the calculations (4 adult and 2 pediatric). In order to achieve a 4-h acetaminophen concentration of 100 mg/L, doses (mg/kg ± 99.9CI) of 180.5 ± 43.2 for adults and 396.1 ± 115.5 for children were calculated.

DISCUSSION

A dose of 75 mg/kg would likely yield a 4-h acetaminophen concentrations well below 100 mg/L. Medical toxicologists and poison information specialists are left without evidence-based guidance for which patients or which ingestion history would now warrant referral to hospital for acetaminophen concentration measurement. Larger toxicokinetic studies in acetaminophen overdose are needed to define ingestion dose for referral to hospital.

Efficacy of ipecac during the first hour after drug ingestion in human volunteers

OBJECTIVE

To determine the decrease of drug absorption when syrup of ipecac is administered at various times within one hour of drug ingestion.

METHODS

Ten healthy human volunteers were recruited for a four-limbed randomized crossover study. The three experimental limbs consisted of administration of 30 mL syrup of ipecac, at 5, 30, or 60 minutes after ingestion of 3900 mg acetaminophen as 12 x 325 mg tablets with 250 mL room temperature water. The fourth limb served as control. Blood samples were drawn at 0, 0.5, 1.0, 2.0, 3.0, 4.0, 6.0, and 8.0 hours after analgesic ingestion for serum acetaminophen concentration determination by high-performance liquid chromatography. Repeated measures ANOVA and Tukey's HSD tests were used for group comparisons.

RESULTS

The area under the serum concentration vs time curve was (mean +/- SD) 206 +/- 48, 67 +/- 37, 183 +/- 78, and 162 +/- 47 mg/L for control, 5, 30, and 60 minutes, respectively. This corresponds to decreases in bioavailability of 67, 11, and 21%. Only the 5-minute group differed significantly from control (p < 0.05). Sedation was observed as a significant adverse effect of ipecac administration.

CONCLUSIONS

Our data do not support benefit from ipecac administration at 30 minutes and beyond. Our data suggest that benefit is lost at some point between 5 and 30 minutes. The sedative effect of ipecac may confound the observation of patients who have ingested sedative hypnotic agents.

Prediction of acetaminophen concentrations in overdose patients using a Bayesian pharmacokinetic model

A pharmacokinetic program using population-based parameter estimates and a Bayesian forecasting model was retrospectively evaluated for predicting acetaminophen serum concentrations in overdose patients. Dynamic disposition factors known to affect acetaminophen disposition (emesis, activated charcoal, N-acetylcysteine, etc.) were included in the program. Twenty six patients who reported an acetaminophen ingestion of at least 70 mg/kg within 24 h of presentation to the hospital and had at least one measured acetaminophen concentration were included. Prediction of initial acetaminophen concentrations using only population-based parameter estimates resulted in a percent mean error (%ME) and percent mean absolute error (%MAE) of 9.3 and 42.2, respectively. Using only the initial concentration as feedback, the Bayesian forecasting model accurately predicted the second acetaminophen concentration (%ME = 4.0, %MAE = 23.6). The Bayesian model also accurately predicted all concentrations within 8 h of the ingestion (%ME = 10.6, %MAE = 24.0). The prediction of concentrations between 2 to 4 h and 4 to 4.5 h after ingestion with only population-based parameter estimates resulted in %ME of 17.0 and 13.2, respectively, and %MAE of 36.5 and 35.1, respectively. Our data suggests that acetaminophen serum concentrations occurring within the first 4.5 h after ingestion can be reliably predicted by the set of population-based parameter estimates evaluated. Once a single acetaminophen concentration is available, the Bayesian forecasting model can accurately predict subsequent concentrations within the first 8 h after an acetaminophen ingestion.

Theophylline Toxicokinetics

Toxicokinetics, the application of pharmacokinetic principles in the assessment and management of the poisoned patient, is demonstrated for theophylline poisonings. Theophylline intoxication and its treatment is briefly reviewed and differentiated for acute or chronic etiologies. A toxicokinetic strategy for the simple calculation and evaluation of serum theophylline concentrations following sustained-release products overdosage is presented. A toxicokinetic basis for treatment durations of activated charcoal, hemoperfusion, or hemodialysis is described. Common toxicokinetic factors responsible for presentation with chronic theophylline intoxication are mentioned.

Clinical evaluation of an acetaminophen meter for the rapid diagnosis of acetaminophen intoxication

STUDY OBJECTIVES

To test the accuracy of a newly developed meter that determines serum acetaminophen concentration after a 30-second analysis of one drop of whole blood.

DESIGN

Sixty-six blood samples from patients with known or suspected drug overdose were analyzed for the presence of acetaminophen. In all cases determination of serum acetaminophen concentration was performed simultaneously with the meter and by a reference laboratory.

SETTING

Eligible patients were those who presented during a nine-month period to the emergency departments of two tertiary care hospitals (including a pediatric hospital).

RESULTS

Thirty-one specimens had a laboratory-confirmed detectable acetaminophen concentration. The meter identified these toxic specimens in all cases; there were no false-negatives (sensitivity, 100%). Among the 35 specimens not containing acetaminophen, the meter invariably confirmed a nondetectable serum acetaminophen concentration (specificity, 100%). Acetaminophen measurements by the meter correlated strongly with laboratory determinations (r = .985, P less than .001). Repeated testing of one specimen documented the precision and reproducibility of the meter's analysis; mean coefficient of variation was .08 in measuring toxic acetaminophen concentrations. Drug coingestion had no significant effect on the accuracy of the meter. Instrument accuracy was maintained after more than 100 uses without recalibration.

CONCLUSIONS

This meter identifies the possibility of rapid and accurate determinations of serum acetaminophen concentration. The instrument is ideally suited for patients with acetaminophen poisoning in whom expeditious and appropriate administration of antidotal therapy is desired.