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Dosing weight

What is dosing weight?​

Medication doses in the underlying formulary may vary based on weight dependence, either because the dose (or dose minimum/maximum or maximum cumulative dose in a given time) is expressed in relative terms (e.g., '/kg') or because doses are categorised by weight bands (refer to Formulary Filtering). Typically, when medication doses are weight-dependent, the (total) body weight of the patient is used to determine the appropriate dose. However, formularies integrated with Touchdose may recommend the use of an alternative body weight scalar, such as ideal body weight (IBW) or adjusted body weight (AdjBW), when calculating doses for obese patients.

note

There are various alternative bodyweight alterations used clinically for obese patients, including but not limited to ideal body weight (IBW), lean body mass (LBM), and adjusted body weight (AdjBW) [12,13]. However, there is a lack of standardised, comprehensive national or international guidance on how the dose should be corrected for each drug and patient group. This is, in part, due to a lack of pharmacokinetic studies on individual drugs in obese patients (e.g., [13,14]). IBW is the most commonly used body weight alteration in obese patients and the only alternative body weight correction used in the British National Formulary [15]. Touchdose supports IBW and AdjBW where the underlying formulary specifically instructs their use.

What dosing weight is used by Touchdose?​

Touchdose utilises total body weight for all dose calculations in non-obese patients and in most weight-based dose calculations for obese patients. An alternative body weight scalar, such as IBW (ideal body weight) or AdjBW (adjusted body weight), is employed only in specific instances when the underlying formulary mandates its use to make dose recommendations for obese patients.

When the formulary necessitates the use of IBW or AdjBW for obese patients, the application will initially check the patient's obesity status before deciding whether to use the alternative body weight scalar for dose calculations. If the system lacks the required patient parameters to distinguish the patient's obesity status, it will prompt the clinician to enter the necessary information. The clinician can choose to enter the patient's height, or alternatively, click to choose that the patient is not obese and so the system to use the patient's body weight. This can be either because it is evident that the patient is not obese or because the clinician wishes to override the guidance stating that an alternative body weight scalar should be used in obese patients. If Touchdose determines that a patient is obese, it can also calculate the relevant alternative body weight scalar since the same inputs are required for both.

In cases where the use of IBW or AdjBW is required, it will be employed to (where relevant):

  • To determine which formulary weight band the patient falls into.
  • Determine the absolute dose from a relative ('/kg') dose.
  • Establish a relative minimum/maximum dose for relative ('/kg') continuous infusions.
  • Determine the absolute dose from a relative ('/kg') maximum cumulative dose in a given time.

For clarity, total body weight will be used for all other calculations in Touchdose that require the patient's weight, including:

Whenever IBW or AdjBW is utilised, Touchdose will notify the clinician by displaying an orange information button at the end of the dose recommendation, known as a smart alert. Clicking on the button will provide an explanation stating which scalar has been used, along with the value that has been used. See Smart alerts for more information and for an example.

Instructing the system that the patient is not obese​

As previously outlined, users can indicate that a patient is not obese, allowing them to bypass entering the patient’s height. In this case, the system will use the patient's total body weight. This decision is saved and applied when the system makes dose recommendations for other medications and conditions. The patient header will be updated to include a note stating the patient is not obese. This note will only appear if the user manually indicates the patient’s obesity status, rather than the system determining it based on the patient’s data. By clicking the 'Not Obese' flag, you can reset this decision. Alternatively, you can enter the patient's height, in which case the system’s obesity determination will override the manual entry. If the user marks the patient as not obese, the patient’s BMI will display as 'Normal'.

Refer to the figure below for an illustration of what the pop-up looks like when the patient's obesity status needs to be known:

obesity status required