Drug dosing in obesity

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Obese and overweight children are on the increase. A common area of confusion is – how we are supposed to calculate their medications given their weight?

How do we define 'overweight'?

In children under 2:

  • Overweight is when weight-for-height values are above the 95th centile

In children over 2 years old:

  • Overweight is a BMI-for-age between the 85th and 95th centiles
  • Obese is a BMI-for-age above the 95th centile

In adolescents:

  • Obese is BMI >95th centile or 30mg/kg/m², whichever is lower

Additionally, Underweight is a BMI-for-age below the 5th centile

And how should we calculate the ideal body weight?

The Moore method uses height-for-age growth charts to give a percentile. That percentile is then used to read the ideal body weight from a weight-for-age growth chart

It is based on the concept that the ideal body weight is the same standard deviation from the mean as the child’s height. So, for example, if a child’s height is on the 95th centile, then you should look at a weight growth chart and work out what the weight on the 95th centile for his age is. That would be his ideal body weight.

This is easy to do, because you only need a growth chart to calculate the idea body weight.

The Moore method has its limitations. In particular it may over estimate the ideal body weight in tall children. Other methods are the McLaren method and the BMI method.

The McLaren method:

  • Plot the height
  • Draw a horizontal line to the 50th centile
  • Look at the age for that 50th centile
  • Look up the 50th centile weight for that age

The BMI method:

Ideal body weight = (BMI at 50th centile for child’s age) x (height in m2)

Why is weight and drug dosing important?

In children with obesity, body fat is not equally distributed, and obese children have a higher percent fat mass and lower percent lean mass. Therefore, calculating drug doses according to total body weight can result in over-dosing.  Conversely, calculating doses based on ideal body weight can be sub-therapeutic.

So, which drugs should be calculated according to ideal body weight and which should be calculated according to actual body weight?

For loading doses, the calculation is based on volume of distribution.

Hydrophilic drugs are based on ideal body weight.

Partially lipophilic drugs are based on an adjusted body metric.

Lipophilic drugs distribute freely into fat and so in obesity and overweight children, a larger dose may be needed. So these drugs may be calculated on total body weight (but bearing in mind toxicity).

Maintenance dose calculations are based on clearance rate. This is detemined by renal and hepatic function and we don’t know the effect of obesity on these functions.

Let’s get down to the specifics.

Antibiotics

Most antibiotics should be calculated on total body weight, including – penicillins (maintenance and loading), cephalosporins, vancomycin (maintenance and loading), carbapenem.

Some require a specific metric of

(0.4[TBW-IBW]) + IBW)

These are ciprofloxacin, gentamicin, amikacin, tobramycin.

For all of these, remember that the total dose should not exceed the adult recommended dose.

Analgesia

Paracetamol, opiates, and ketamine should all be dosed to ideal body weight.

If the ideal body weight is >40kg then it’s best to use adult dosing. The exception to this may be paracetamol where mg/kg dosing can be used, unless the patient is >65kg (see comments section for more details on this).

Anticonvulsants

Phenytoin, carbamazepine and benzodiazepines should all be dosed to ideal body weight. The exception is that for a phenytoin loading dose, use the metric (1.33[TBW-IBW]) + IBW).

 

References

Kendrick JG, Carr RR, Ensom MHH. Pharmacokinetics and Drug Dosing in Obese Children. The Journal of Pediatric Pharmacology and Therapeutics : JPPT. 2010;15(2):94-109.

Phillips S, Edlbeck A, Kirby M, Goday P. Ideal body weight in children. Nutr Clin Pract. 2007 Apr;22(2):240-5.

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About 

Tessa Davis is a paediatric emergency registrar from Glasgow and Sydney, but currently living in London. Tessa tries to spend time with her 3 kids in between shifts. @tessardavis | + Tessa Davis | Tessa’s DFTB posts

Author: Tessa Davis

Tessa Davis is a paediatric emergency registrar from Glasgow and Sydney, but currently living in London. Tessa tries to spend time with her 3 kids in between shifts. @tessardavis | + Tessa Davis | Tessa’s DFTB posts

2 Responses to "Drug dosing in obesity"

  1. Neil Hughes
    Neil Hughes 2 years ago .Reply

    Great article Tessa, lots of useful stuff. I just wanted to sound a note of caution about using adult dosing for paracetamol in older children. We have been guilty in the past of inadvertently over-dosing adults with low body weight and indeed there was a tragic death here in Scotland a few years ago (with IV paracetamol, but the same principle applies) . I’d still calculate as mg/kg until the patient is north of 65kg.

    • Tessa Davis
      Tessa Davis 2 years ago .Reply

      Thanks Neil. Good point re adult dosing for paracetamol. I’ve amended the article to reflect this.

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