Derived patient parameters
Age and age classification
Touchdose calculates the age of the patient in years, months and days from the birth date provided.
Touchdose classifies patients in one of the following categories:
| Category | Age range |
|---|---|
| Neonate | 0 days up to 28 days |
| Child | 28 days up to 18 years |
| Adult | 18 years up to 65 years |
| Elderly | 65 years and older |
The top end of the range is not included i.e. a patient with an age of 28 days will be classified as a child.
This age classification is for the purposes of Touchdose providing an age category to the clinician. Elsewhere in this document, references to adults apply to patients 18 years and older while references to children apply to patients below 18 years.
Body mass index and percentiles
Touchdose utilises the patient's body mass index (BMI), measured in kg/m2, to determine whether an adult is clinically obese or not. A child is classified as clinically obese based on their BMI percentile.
The BMI is calculated using the formula: BMI = (body weight) / (body height)2, where body weight is in kilograms (kg) and body height in metres (m).
Touchdose checks that the calculated BMI falls within acceptable bounds, as defined below:
- For adults, the BMI must be > 12kg/m2 and < 50kg/m2.
- For children, the BMI must be within the 0.01th and 99.99th sex-specific BMI-for-age percentiles, including the boundary values.
BMI percentiles are exclusively calculated for children, and the method employed is the LMS method.
- Touchdose adheres to guidance from the Royal College of Paediatrics and Child Health Growth Charts for data validation. The Royal College of Paediatrics and Child Health (RCPCH) specifies the use of different growth charts for various patient ages as follows:
- Children aged 2 weeks to 3 years and 11 months: World Health Organisation growth standard data
- Children aged 4 years to 18 years (excluding 18 years): UK1990
- In the absence of RCPCH guidance for children under 2 weeks, Dosium has opted to use UK1990, given its inclusion of data for those patients and its reference by the RCPCH for children of other ages.
- Touchdose does not provide support for special validation rules for children with alterations to expected growth, such as ex-premature children or children with Down syndrome. Any children with calculated BMI values outside of the percentile ranges provided above will fail validation and result in an error which means that Touchdose cannot be used for that patient.
- As detailed in Limitations and Contraindications, Touchdose does not discriminate medication suggestions based on pregnancy status. Clinicians who, at their discretion, choose to use medication suggestions from Touchdose in pregnant patients, despite the Contraindications and Warnings, should be aware that such suggestions will be based on whatever weight is passed to Touchdose as body weight.
- BMI values calculated by Touchdose based on body weight and height values received from the EHR may not be suitable for pregnant patients. This may lead to Touchdose erroneously indicating that the patient is clinically obese and may result in unsuitable dosing suggestions.
Obesity status
A patient's obesity status is required when Touchdose provides patient-input-specific doses for which the underlying formulary recommends the use of an alternative body weight scalar, such as ideal body weight (IBW) or adjusted body weight (AdjBW), in obese patients (refer to Dosing Weight).
When the formulary requires information about a patient's obesity status, and the system does not have enough data to determine this, Touchdose will prompt the clinician to provide the necessary details. The clinician can either confirm that the patient is not obese, allowing the system to use total body weight for dose calculations (refer to [Instructing the system that the patient is not obese](/docs/Core Resources/dosing-weight#instructing-the-system-to-use-total-body-weight) for more information), or if unsure of the patient's obesity status, they can choose to have Touchdose calculate the BMI by entering the patient's height. The figure below shows how the system gathers information to assess obesity status:
Touchdose categorises patients into weight categories as follows:
For adult patients (18 years and older), Touchdose adheres to NHS guidelines and classifies patients based on their BMI:
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy | Between 18.5 and 24.9 |
| Overweight | Between 25 and 29.9 |
| Obese | 30 and above |
For children, Touchdose determines clinical obesity based on the patient's sex-specific BMI-for-age percentiles. The weight categories are as follows:
| Category | Sex specific BMI-for-age percentile range |
|---|---|
| Underweight | Below 2nd |
| Healthy | Between 2nd and 90.9th |
| Overweight | Between 91st and 97.9th |
| Obese | 98th and above |
- For Adults (>= 18 years), Touchdose follows the NHS guidelines, where BMI values 30 and over are considered clinically obese.
- For children 2 weeks and over, clinical obesity is based on patient BMI percentiles which in turn varies with sex and age. The exact definition for child obesity, or more specifically when to account for obesity when determining doses, has not been established across all key stakeholders. Touchdose follows the definition provided in the UK Neonatal and Paediatric Pharmacists Group 2018 report which states that a child is obese if their BMI-for-age is above the 98th percentile. The RCPCH BMI-for-age growth charts are used when determining whether a child is clinically obese. These charts combine data from the WHO 2006 and from the British 1990 (UK90) growth reference charts. Note that the RCPCH BMI-for-age data do not cover the period birth to 2 weeks but the decision has been made to use the UK90 growth charts for this age range.
Ideal body weight
Ideal body weight (IBW) may be recommended by the underlying formulary when determining a patient-specific dose in certain circumstances, such as in obese patients (see Dosing Weight). Touchdose employs different methods for calculating IBW in adult patients and paediatric patients.
- For adults, Touchdose uses the body mass index (BMI) method, as recommended by [1]. This method sets the IBW as the weight corresponding to an ideal body mass index of 22 kg/m2: IBW = 22 x (body Height)2
- For children, Touchdose uses the BMI-50 formula, as recommended by [2]. According to this method, IBW = (50th percentile sex-specific weight for age) x (body height)2
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Various alternative methods exist for calculating ideal body weight.
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For adults, one commonly used alternative method is the Devine formula [3]. Touchdose opts for the BMI method over the Devine formula for ideal body weight (IBW) calculations. This choice is based on the fact that the Devine formula produces IBW values with varying implied Body Mass Index (BMI) for patients of different heights. For dosing modifications proposed on the basis of compartment theory, the BMI method is deemed more consistent [4].
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In paediatrics, several methods, including the Moore [5] and McLaren [6] methods, are available for calculating IBW. Previous research [7] comparing these methods showed good correlation at the 50th percentile through age 18 years and across all percentiles in children younger than 8 years old. However, these three methods yielded widely divergent results for IBW at the lowest and highest percentiles in patients aged 8 to 18 years. Differences across methods were also reported in [8], demonstrating increased discrepancies in children over 25 kg. Despite these known variations, there is no consensus on which formula should be applied to which patients. While the British National Formulary for Children recommends the use of IBW for certain drug doses, it does not declare a preference for a specific IBW calculation method. Guidance from the Neonatal and Paediatric Pharmacists Group (NPPG) in a 2018 report 'How should medicines be dosed in children who are obese?' [2] suggests using the BMI-50 method for pragmatic reasons, as it can be consistently applied across most paediatric age groups. Touchdose follows this recommendation.
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The 50th percentile sex-specific data are derived from the same World Health Organisation and UK1990 growth standard data used in the Royal College of Paediatrics and Child Health Growth Charts.
Due to the existence of various methods for calculating ideal body weights, there is potential for disagreement with Electronic Prescribing and Medicines Administration (EPMA) systems that calculate IBW using different methods. Touchdose will use its own calculated IBW for dosing calculations and will not retrieve an IBW value from any EPMA system. Caution is advised.
Adjusted body weight
Adjusted body weight (AdjBW) may be recommended by the underlying formulary when determining a patient-specific dose in certain circumstances, such as in obese patients (see Dosing Weight). Touchdose only calculates AdjBW when the patient is classified as obese and the formulary requires it.
- For adults, Touchdose calculates adjusted body weight using the following formula: AdjBW = IBW + 0.4 x (TBW - IBW)
- For children, Touchdose calculates adjusted body weight using the following formula: AdjBW = IBW + 0.35 x (TBW - IBW)
When shown in the patient header, the calculation method for AdjBW is described as:
- For adults:
0.4 correction - For children:
NPPG 0.35 correction
Touchdose calculates AdjBW from its own derived IBW and total body weight values. It does not retrieve an AdjBW value from any EPMA system. Caution is advised as other systems may use different formulae or labels.
Weight and height percentiles for children
For paediatric patients (under 18 years), Touchdose calculates, when provided with the necessary patient inputs, the weight-for-age and height-for-age percentiles. The weight and height percentiles are used to validate patient’s weight and height, and only those falling between the 0.01th and 99.99th sex-specific percentiles are accepted. If an incorrect value that fails validation is entered, a new valid input is required before Touchdose can be utilised. If the patient's accurate weight or height fails validation, Touchdose cannot be used for that particular patient.
- Touchdose uses the same World Health Organisation and UK1990 growth standard data used in the Royal College of Paediatrics and Child Health Growth Charts for weight-for-age and height-for-age validation.
- Touchdose derived weight-for-age and height-for-age percentiles are calculated using the LMS method.
- Touchdose does not provide support for special validation rules for children with alterations to expected growth, for example, ex-premature children or children with Down syndrome.
Calculated body surface area
Touchdose employs different methods for calculating BSA in adult and paediatric patients.
For Adults and children over 90kg, Touchdose calculates BSA based on the Dubois and Dubois formula [9], which requires knowledge of the patient's body height and body weight. The Dubois and Dubois BSA formula is: BSA = 0.007184 x height(cm)0.725x weight(kg)0.425.
For children 90kg and under, Touchdose calculates paediatric BSA values by following the approach outlined in the British National Formulary for Children (as of October 2021): Touchdose uses the BNFC weight-to-BSA tables, which are based on the Sharkey method [10], itself derived from the Boyd equation. When the body weight of the patient does not exactly match a weight value in the table, Touchdose determines the BSA by interpolating between the nearest body weight measurements.
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Unlike many other BSA formulae, the Sharkey method recommended by the BNFC does not include height. In the original publication the following disclaimer is provided: “although this approach has been shown to be equivalent to other currently used methods, care should be taken extending this calculation of BSA to children less than 10 kg, to obese patients and to those with cachexia”.
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A 2017 NPPG report from University College Hospital, London, UK [11] compared BSA calculations using the Boyd's formula / BNFc nomogram, relying only on the patient's weight, and the Dubois and Dubois formula which considers height. The authors suggested that "measuring BSA in Young People (by each of the methods above) gives estimates within 10% for patients with a BMI within the normal range and up to 20% in patients that are obese or underweight. Calculations involving height may provide better estimates." Touchdose follows the recommendation of the BNFC and uses the weight-based tables.
Clinicians should be aware that paediatric BSA determination is performed by Touchdose using the Sharkey method which does not include patient length in the calculation. No modifications are made to Sharkey-determined BSA values or doses calculated based upon these values in children under 1, or those at the extremes of habitus.
Because there are different ways to calculate BSA, there is the potential for the BSA calculated by Touchdose to differ with EP Systems that use different methods to calculate BSA. BSA calculated by Touchdose can be seen in the patient header and will be used for dose calculations. Caution is advised.
Creatinine reference range
Whenever creatinine is retrieved from the EHR, the system will also display the corresponding reference range. These reference ranges are based on age and sex, determined using Caliper using the Enzymatic lab test and the Abbott Alinity lab instrument as of 21/03/2025, and are listed below:
Females
| Age | Lower limit | Upper limit |
|---|---|---|
| 0-15 days | 29 | 82 |
| 15 days - 2 years | 9 | 32 |
| 2 - 5 years | 18 | 38 |
| 5 - 12 years | 27 | 54 |
| 12 - 15 years | 40 | 72 |
| 15 - 19 years | 43 | 74 |
Males
| Age | Lower limit | Upper limit |
|---|---|---|
| 0-15 days | 29 | 82 |
| 15 days - 2 years | 9 | 32 |
| 2 - 5 years | 18 | 38 |
| 5 - 12 years | 27 | 54 |
| 12 - 15 years | 40 | 72 |
| 15 - 19 years | 55 | 96 |
Please note that all the age intervals above are up to but not including the upper bounds.
Creatinine is not retrieved for adults.
If a patient’s creatinine level falls outside the reference range, the system will clearly indicate it as High or Low.
eGFR
Touchdose does not calculate eGFR for adults in the current release. The eGFR calculation described below applies to paediatric patients only.
We use the Modified Bedside Schwartz equation to calculate the Estimated Glomerular Filtration Rate (eGFR).
To perform this calculation, we require a valid and up-to-date serum creatinine measurement and height.
Neonates (≤1 month old):
eGFR (mL/minute/1.73m2) = 30 x height (cm) / serum creatinine (micromol/litre)
Children (>1 month old):
eGFR (mL/minute/1.73m2) = 35 x height (cm) / serum creatinine (micromol/litre)
Important Considerations:
- The Modified Bedside Schwartz equation requires a stable creatinine level and is inaccurate when serum creatinine is changing rapidly.
- It is not suitable for children with reduced muscle mass, amputations, cachexia, or those taking creatine supplements.
- This formula does not apply to children receiving dialysis or renal replacement therapy.
eGFR category
Based on the calculated eGFR we then provide a category according to KDIGO.
| eGFR value (mL/minute/1.73m2) | Description | Category |
|---|---|---|
| Greater than or equal to 90 | Normal or high | G1 |
| 60-90 | Mild reduction | G2 |
| 45-60 | Mild-moderate reduction | G3a |
| 30-45 | Moderate-severe reduction | G3b |
| 15-30 | Severe reduction | G4 |
| 0-15 | Kidney failure | G5 |
It is crucial to understand that Touchdose does not factor eGFR into dosing recommendations. Failing to account for this and not exercising appropriate caution when adjusting doses or medications may lead to patient harm.
eGFR values in infants under 6 months of age should be interpreted with caution. Estimated glomerular filtration rate (eGFR) calculations are less reliable in neonates and young infants due to the ongoing postnatal maturation of renal function, lower and variable muscle mass, and differences in creatinine handling. In this age group, eGFR may not accurately reflect true renal function and should not be used in isolation to guide clinical decisions. Where renal impairment is suspected, further assessment (e.g., serial creatinine measurements, urine output monitoring, or specialist input) is advised.
There are other methods that can be used to calculate eGFR such as Cockroft-Gault or CKD-EPI but Modified Bedside Schwartz has been deemed the most suitable calculation method for children.
Caution renal function flag
A renal caution flag will be displayed in the patient header based on the following criteria:
- if the eGFR is below 45 mL/min/1.73m², indicating a moderate-to-severe reduction (G3b), severe reduction (G4), or kidney failure (G5).
- If eGFR cannot be calculated due to missing height, the flag will appear if the creatinine level exceeds the reference range. You will be prompted to enter the patient’s height to enable eGFR calculation. If the calculated eGFR is 45 mL/min/1.73m² or higher, the flag will be removed.