Abdominal sagittal diameter (ASD) is the distance from the lower back to the highest point of the abdomen. It measures the anteroposterior diameter of the abdomen.
A simple and inexpensive method for the assessment of abdominal obesity, ASD has been shown to have strong associations with glucose intolerance, cardiovascular risk and mortality independent of other anthropometric measures.
ASD measurement can be combined with other anthropometry to predict visceral fat and subcutaneous fat using published validated equations.
ASD is commonly measured using an abdominal caliper with a moveable arm, such as the Holtain-Kahn Abdominal Caliper. Calibration of the caliper should be carried out at regular intervals, using rods of predetermined lengths or an instrument similar to this calliper checking gauge. ASD can also be measured using a CT or MRI scan (the gold standard for measuring different fat compartments); however these methods are limited to practical and ethical constraints. Laser beam devices have also been used in children and adolescents, a technique much favourable in these groups as no body contact is required.
Protocol using abdominal caliper
Figure 1 Measuring abdominal sagittal diameter.
Source: NHANES Anthropometry Procedures Manual (2013). 
Abdominal sagittal diameter is typically used to estimate the amount of visceral fat in both clinical setting and within epidemiological research. It is a simpler, cheaper and less invasive (while still reliable) method than CT and MRI.
Abdominal thickness measurements are typically recorded in cm, and often the raw values are used on their own as they act as an indicator of total abdominal fatness. However, they can be combined with other anthropometric measures (including waist circumference, abdominal diameter, hip circumference and skinfolds) and entered into validated equations to estimate visceral and or subcutaneous fat. A circumference tape can be used for the waist, hip and thigh measurements and skinfold calliper for the skinfold thickness.
Several published equations are available; some of the most commonly used are listed below.
Pinter et al. (2017)
Samouda et al. (2013)
He et al. (2004) based on MRI measurements of ASD
Raw and derived estimates should be quality checked during data processing in the same manner as other health related variables, for example checking for outliers and data entry errors.
An overview of the characteristics of abdominal sagittal diameter methods is outlined in Table 1.
Table 1 Characteristics of abdominal sagittal diameter methods.
|Number of participants||Large|
|Relative cost||Low. However more expensive than taking the waist circumference.|
|Researcher burden of data collection||Low|
|Researcher burden of coding and data analysis||Low|
|Risk of reactivity bias||No|
|Risk of recall bias||No|
|Risk of social desirability bias||No|
|Risk of observer bias||Yes|
|Suitability for field use||High|
|Participant literacy required||No|
Considerations relating to the use of abdominal sagittal diameter methods in specific populations are described in Table 2.
Table 2 Use of abdominal sagittal diameter methods in different populations.
|Pregnancy||Not suitable as we cannot differentiate between mother's tissue and fetal tissue with this measurement alone.|
|Infancy and lactation||Suitable, but potentially difficult to obtain accurate measures as this population is unlikely to refrain from movement during measurements.|
|Toddlers and young children||Suitable, but potentially difficult to obtain accurate measures as this population is unlikely to refrain from movement during measurements.|
|Other (obesity)||Suitable. The caliper with a larger gap is typically used. Easier to measure in severe obesity as the measurement is made while lying down and issues with pendulous abdominal adipose panniculus (dense layers of subcutaneous fat tissue in the lower part of the abdomen) are reduced.|
It is recommended to estimate the technical error of measurement to monitor intra- and inter-observer variation.
Refer to section: practical considerations for objective anthropometry
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