Disagreement Between Height/Weight Classifications of Underweight, Normal Weight, and Obesity in Peritoneal Dialysis Patients

Antonios H. Tzamaloukas,1 Glen H. Murata,1 James E. Hill,2 Annette Leger,2 Linda Macdonald,1 Sarah Baron,1 Richard M. Hoffman1
From: 1Medicine Service, New Mexico Veterans Affairs Health Care System, and Department of Medicine, University of New Mexico School of Medicine; and 2Dietary Service, New Mexico Veterans Affairs Health Care System, Albuquerque, New Mexico, U.S.A.

Peritoneal dialysis (PD) patients are classified as underweight, normal weight, or obese by height/weight indices including body mass index (BMI) and the body weight/desired weight (W/DW) ratio. We compared these classifications of degree of obesity in 378 women and 555 men on PD. We used these cut-off values: for underweight, BMI £ 18.5 and W/DW £ 0.9; for obesity, BMI ³ 30.0 and W/DW ³ 1.2. The W/DW values were calculated assuming first a small frame, then a medium frame, and finally a large frame for all subjects.
Regardless of sex or skeletal frame, BMI correlated highly with W/DW (r value between 0.98 and 0.99); however, the range of BMI values corresponding by linear regression to the normal range of W/DW (0.9 – 1.2) was narrower than the range of “normal” BMI (18.5 – 30.0). Consequently, regardless of sex or skeletal frame, smaller fractions of the patient population were classified as underweight or obese by BMI standards than by W/DW standards.
The degree of agreement of the classifications of subjects as underweight, normal weight, or obese by BMI and W/DW was evaluated by Cohen’s kappa ratio. The kappa ratio varied between 0.47 and 0.58, indicating a reasonable—but not high—degree of agreement beyond chance. The highest kappa ratios were obtained assuming a medium skeletal frame for both women and men.
Substantial discrepancies are observed in the classification of PD patients as underweight, normal weight, or obese by BMI and W/DW. Further research is needed to identify the height/weight index that has the strongest association both with clinical outcomes and with other, more precise measurements of body fat content.

Key words

Introduction

Weight deficit (1) and obesity (2,3) may both affect the clinical outcome of peritoneal dialysis (PD). The definition of underweight and obesity in PD patients is not clear. Jones (1) analyzed the results of an international cross-sectional study of nutrition in continuous ambulatory peritoneal dialysis (CAPD) patients, assuming that all patients in the study had a medium skeletal frame. He defined underweight as body weight lower than the desired weight (DW). Desired weight is the midpoint in the range of body weights associated with the greatest longevity for normal individuals in the same age range and with the same height, sex, and skeletal frame as the individual in question (4). Desired weight is obtained from the actuarial tables of the Metropolitan Life Insurance Company (5).
The ratio of body weight to DW (W/DW) can be used to classify subjects as underweight, normal weight, and obese. The following W/DW cut-offs can be applied in renal patients (6): underweight, W/DW £ 0.9; normal weight, 0.9 < W/DW < 1.2; obesity, W/DW ³ 1.2.
Recent National Institutes of Health (NIH) guidelines (7) classify the degree of obesity by body mass index (BMI). The following BMI cut-off values were proposed (7): underweight, BMI £ 18.5; normal weight and overweight, 18.5 < BMI < 30.0; obesity, BMI ³ 30.0. (W/DW guidelines make no distinction between normal weight and overweight. In its comparisons, this report collapses normal weight and overweight BMI categories into a normal weight category.)
Desired weight and body mass index are both estimates of weight per unit height. They are, therefore, called “height/weight” indices (8). We previously reported discrepancies in the cut-off values for underweight and obesity between BMI and ideal weight, which is another height/weight index (9). The present report evaluates the agreement or disagreement in the classification of degree of obesity of PD patients by BMI and W/DW.

Patients and Methods

We analyzed the relationship between BMI and W/DW and between the classifications of subjects as underweight, normal weight, or obese by BMI and by W/DW in 933 PD patients. Weights recorded were at or near the clinically determined “dry” weight. Values for DW were obtained from the Metropolitan Life Insurance DW tables (5). The skeletal frame of individual patients was not known. We therefore calculated W/DW values assuming first a small frame, then a medium frame, and finally a large skeletal frame for all subjects.
The relation of BMI to W/DW was analyzed by linear regression, where BMI was the dependent variable. From the regressions, we obtained the BMI values corresponding to the W/DW cut-off values of 0.9 for underweight and 1.2 for obesity. We also calculated the 95% confidence intervals of these BMI values. We compared the intervals to the BMI cut-off values of 18.5 for underweight and 30.0 for obesity.
Finally, using the cut-off values described in Introduction, we compared the classifications of subjects as underweight, normal weight, or obese by BMI and W/DW. For this comparison, we used Cohen’s kappa ratio (10). A kappa ratio below 0.40 indicates a low degree of agreement; a kappa ratio in the range 0.40 – 0.75 indicates a reasonable degree of agreement; and a kappa ratio above 0.75 indicates a high degree of agreement beyond chance. Classification of degree of obesity by W/DW was performed three times for each subject, assuming first a small skeletal frame, then a medium frame, and finally a large frame. All comparisons were performed separately in women and men.

Results

Table I shows pertinent patient characteristics. Figure 1 shows the relationship between the medium-frame W/DW and the BMI in women and in men. Table II shows the regressions of W/DW on BMI. Values under the columns W/DW = 0.9 and W/DW = 1.2 in Table II represent the BMI values (95% confidence intervals) predicted from the corresponding regressions for W/DW = 0.9 (underweight W/DW cut-off) and W/DW = 1.2 (obesity W/DW cut-off), respectively. Regardless of sex or skeletal frame, the lower 95% confidence limits of the BMI values corresponding to a W/DW value of 0.9 were larger than 18.5. The 95% confidence intervals of the BMI values corresponding to a W/DW value of 1.2 contained the BMI value of 30.0 only in large-frame women and men. For women and men of small and medium frame, a BMI of 30.0 was larger than the upper 95% confidence limit of the BMI values corresponding to a W/DW value of 1.2.
Table III shows the classification of women and men as underweight, normal weight, or obese by BMI and W/DW for a medium skeletal frame. Only 33 of the 74 women and only 15 of the 77 men classified as underweight by W/DW had a BMI £ 18.5. Also, only 80 of the 132 women and 87 of the 141 men classified as obese by W/DW had a BMI ³ 30.0. Table IV shows the kappa ratios between the classifications of the degree of obesity by the BMI and W/DW standards. Kappa ratios were between 0.47 and 0.58, indicating a reasonable—but not high—degree of agreement beyond chance. The highest kappa ratios were obtained assuming a medium skeletal frame for both women and men.

figure 1 Regression of body weight/desired weight (W/DW) on body mass index (BMI) in women (upper panel) and in men (lower panel), assuming medium skeletal frame.

Discussion

Conclusion

Classifications of degree of obesity of PD patients by the BMI and W/DW standards show substantial disagreement. To test whether one of the height/weight indexes has more advantages than the other, prospective comparisons of clinical outcome by the two indices are needed, as are studies correlating the two indices to more precise methods of evaluating obesity.

Acknowledgment

This work was supported by the New Mexico VA Health Care System.

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Corresponding author:

Antonios H. Tzamaloukas, md, Renal Section (111C), New Mexico VA Health Care System, 1501 San Pedro, SE, Albuquerque, New Mexico 87108 U.S.A.