Variability in ideal body weight formulae

Dear Sir,

The concept of ideal body weight (IBW) has been used since at least 1871, when Pier Broca, a French surgeon, wrote in his book Mémoires d'anthropologie that a simple method to calculate IBW is by subtracting 100 from a patient's height when measured in centimetres. Since then, a number of other eponymous formulae have been developed, including Robinson, Devine, Hamwi, Miller, and Hammond, which have been cited and discussed recently. ¹

The use of IBW remains pertinent in modern intensive care medicine and the current COVID-19 outbreak, and a recent paper in JICS ² discusses how IBW or lean body weight, rather than actual body weight may be useful for drug dosing, renal replacement dosing, and nutrition prescriptions. In addition, the recommendation of a tidal volume in acute respiratory distress syndrome (ARDS) of less than 8 ml.kg⁻¹ predicted body weight was based on calculations using the Devine formula, often used for IBW. ³

However, these formulae produce IBWs that are different to the weight calculated by the Devine formula and different to each other, which we wanted to highlight. The average height of an adult in the UK is 175.9 and 161.2 cm for males and females, respectively, and three standard deviations produce a range of 153–198 cm and 141–184 cm, respectively ⁴ ; many of these formulae are only recommended if the patient's height is over 5 ft (152 cm). Using the different formulae, the difference in IBW between Devine and other formulae are up to 14% in males and 19% in females, and greater than 5% in males and 13% in females at every height, as shown in Table 1 and Figures 1 and 2.

Figure 1.

Difference in calculated IBW compared with Devine formula (males).

Figure 2.

Difference in calculated IBW compared with Devine formula (females).

Table 1.

Variability in calculated ideal body weight (kg) by the use of different formulae.

	Hgt (cm)	150	155	160	165	170	175	180	185	190	195	200
	Hgt (inch)	59.06	61.02	62.99	64.96	66.93	68.90	70.87	72.83	74.80	76.77	78.74
Male	Robinson	50.20	53.94	57.69	61.43	65.17	68.91	72.65	76.39	80.13	83.87	87.61
	Devine	47.83	52.35	56.88	61.41	65.94	70.46	74.99	79.52	84.05	88.57	93.10
	Broca	50.00	55.00	60.00	65.00	70.00	75.00	80.00	85.00	90.00	95.00	100.00
	Hamwi	45.50	50.86	56.21	61.57	66.93	72.28	77.64	83.00	88.35	93.71	99.07
	Miller	54.87	57.64	60.42	63.19	65.97	68.75	71.52	74.30	77.07	79.85	82.62
	Hammond	48.00	53.50	59.00	64.50	70.00	75.50	81.00	86.50	92.00	97.50	103.00
Female	Robinson	47.39	50.74	54.09	57.43	60.78	64.13	67.47	70.82
	Devine	43.33	47.85	52.38	56.91	61.44	65.96	70.49	75.02
	Broca	50.00	55.00	60.00	65.00	70.00	75.00	80.00	85.00
	Hamwi	43.21	47.67	52.14	56.60	61.06	65.53	69.99	74.45
	Miller	51.81	54.49	57.17	59.85	62.52	65.20	67.88	70.56
	Hammond	45.00	49.50	54.00	58.50	63.00	67.50	72.00	76.50

Hgt: height.

We wish to highlight the differences in IBW as calculated by different formulae and the variation in drug, renal and ventilation prescriptions that may follow, especially if the methods are used interchangeably. While the concept of ideal body weight is useful, perhaps the variability in the calculation of it is less than ideal.