[Clinical value of estimated half-life in paracetamol poisoning as a complement to Rumack's nomogram]

Rumack's nomogram is usually used to indicate the treatment with N-acetilcysteine in the paracetamol poisoning, but it has several limitations. Paracetamol half-life elimination (t1/2) is approximately of 2 h with therapeutic doses and it increases to more than 4 h in patients with hepatotoxicity. The aim of this study was to determine the usefulness of estimated paracetamol t1/2 as greater than or inferior to 4 h by using a simple ratio in relation to the development of hepatotoxicity.

21 patients with paracetamol overdose were admitted to Son Dureta Hospital (Palma de Mallorca) and Clínic Hospital (Barcelona) over 13 months. The estimated t1/2 is calculated using the quotient between 2 plasma paracetamol concentrations separated by 2 or more hours.

We found a significant difference (p < 0.005) between the group with hepatotoxicity (n = 3; t1/2 = 8,5 h; range: 3,6 - 8,7 h); and without hepatotoxicity (n = 18; t1/2 = 2,4 h; (range: 1,6 - 4,3 h). We observed an agreement between positive ratio and a t1/2 > 4 h, and negative ratio with t1/2 < 4 h, bearing in mind that the quotient is obtained through mathematical equations.

Rumack's nomogram should be complemented with t1/2 estimation in all cases of paracetamol poisoning, especially with those patients for whom we are not able to determine the time of ingestion at presentation or if there has been a multiple-timepoint ingestion.