July 1, 2014: How are minimum observation times determined?

The Tennessee Poison Control Center (TPC) receives approximately 45 new calls from healthcare professionals each day.  “How long do we need to watch the patient?” represents one universal question often asked by these healthcare providers.  So, how does the TPC devise its recommendations for minimum observation timeframes?  The recommendations stem from three main factors: the patient (including age and comorbidities), the substances involved (including coingestants) and their pharmacokinetic properties, and the clinical experiences of the toxicology experts.  Symptomatic patients obviously require continued observation and treatment, but asymptomatic patients, who reach the recommended minimum monitoring time period without displaying symptoms, can be safely cleared and discharged.


Just as no two patients are exactly alike, neither are their reactions to potential toxins exactly alike. For most products, a grey area exists regarding the precise onset and the overall extent of symptoms due to the variability in individual patient responses.  The effects generated by a substance can differ based on the patient’s age, genetic composition, ethnicity, recent food or fluid intake, underlying health issues, and naivety to the product.  Additionally, the exposure scenario and the presences of other therapies can also affect the progression of symptoms.  Each of these aspects may alter the response time to a product by affecting its absorption, distribution, metabolism, and elimination rates, thereby conceivably shortening or lengthening the minimum observation times recommended.  In certain situations, the availability of a responsible person, who can watch the patient after leaving the medical facility, will also influence the observation period.  Although each individual arrives with his or her own unique set of circumstances, it is the substances to which the patient was exposed that have the greatest impact in determining minimum observation time intervals.


All products possess uniquely distinctive pharmacokinetics when introduced into or onto the body.  The pharmacokinetics of a specific substance can fluctuate depending on the amount of the product, the product’s formulation, the route of administration, the naivety of the body to the product, and the co-introduction of other substances.  Pharmacokinetics are affected by the form (solid,  liquid, or  gas), route of exposure (inhaled, injected, ingested, aspirated, or applied to the skin), configuration (crushed, chewed, or whole), preparation (immediate release or timed release such as extended release or sustained release), pH (acid or alkaline), duration of exposure (acute or chronic), and the risk of any interactions with co-ingestants .  With massive exposure amounts, the typical pharmacokinetics of a product can become irrelevant and toxicokinetics supervene. 


Keeping the preceding factors in mind, the Poison Specialists must then examine three important elements of a particular substance’s kinetics when formulating minimum observation timeframes: the peak plasma concentration time, the tissue distribution rate, and the possibility of active metabolites.  Peak plasma concentrations frequently parallel the onset of symptoms and must be considered in establishing a minimum observation time period.  However, peak plasma concentration values represent the average time interval for the plasma concentrations to crest based on prior product studies; thus, time intervals for some patients’ peak levels will be longer than the average value.  Consequently, the recommended minimum observation times will fall beyond the average peak plasma concentration times in order to safely monitor all individuals.  In addition, numerous substances do not distribute instantaneously to all tissues, and peak plasma concentration values may not reflect peak tissue concentrations.  Some symptoms produced by elevated concentrations at the tissue sites may occur long after the plasma concentrations peak; therefore, the Poison Specialists must assess the potential for delayed tissue concentrations when determining minimum observation timeframes.  Production of an active metabolite must also be evaluated.  For some products, the active metabolites are more toxic than the parent compound, and the interim required for the active metabolite formation will alter the minimum observation times.  It is important to remember that the pharmacokinetics of a substance will differ from patient to patient.


One final factor that affects TPC’s minimum observation timeframe recommendations arises from the experiences of those individuals in the toxicology field.  Numerous world-wide studies, both past and present, provide an ever-growing compilation of information regarding the potentially injurious outcomes from a myriad of substances.  These studies look specifically at how variations in patient demographics or how alterations in product introduction influence the product’s effects.  Poison Specialists strive to remain abreast of new research findings to provide the most up-to-date information to other healthcare professionals.  Furthermore, the numerous years of combined personal experiences of the TPC staff gives rise to many valuable previous encounters with a countless number of substances.  These clinical experiences, along with the studies in the toxicology field, assist in the determination of minimum observation times.


Healthcare providers may note that the Poison Specialists often recommend minimum observation time intervals ranging between four to six hours in length based on the above factors.  Many common substances will elicit side effects within a four to six hour timeframe.  Of course, countless products do not fall within the four to six hour window, necessitating either shorter or longer monitoring periods.  In addition, various products may be “Non-Toxic” (depending on the amounts involved and/or the exposure scenario) and do not require any observation or treatment. However, as previously stated, every patient and every event characterizes a unique situation, so calling the TPC for advice regarding minimum observation timeframe recommendations denotes the prudent choice for the healthcare professional.


This Question prepared by:  Cheri Wessels, BSN, MBA, CSPI  (Certified Specialist in Poison Information)


I am interested in any questions you would like answered in the Question of the Week.  Please email me with any suggestion at donna.seger@vanderbilt.edu


Donna Seger, MD
Medical Director

Tennessee Poison Center


Poison Help Hotline: 1-800-222-1222