Within- and Between-Day Reliability of Laboratory-Based Body Composition Assessment Methods.

This study examined the within- and between-day reliability of commonly employed laboratory-based body composition assessment methods. Eighteen healthy adults reported to the laboratory on two occasions, separated by 1-2 days, and were assessed with multiple methods under standardized conditions. These included two dual-energy X-ray absorptiometry (DXA) scanners with manual or automated regions of interest, air displacement plethysmography (ADP) with measured or estimated thoracic gas volume, two multi-frequency bioelectrical impedance analyzers (MFBIA), single-frequency BIA (SFBIA), and bioimpedance spectroscopy. Within-day reliability was established via immediate test-retest assessments at the first visit (technical error), and between-day reliability by repeating assessments at the second visit (technical plus biological error). Within- and between-day measures were evaluated using technical error of the measurement (TEM) and intraclass correlation coefficients (ICCs). For body fat percentage, within-day TEMs ranged from 0.03-0.70%, with ICCs of 0.993-1.000, while between-day ranged from 0.37-1.24%, with ICCs of 0.965-0.998. For fat and fat-free mass (FM and FFM), within-day TEMs ranged from 0.02-0.53 kg, with ICCs of 0.998-1.000 for FFM and 0.992-1.000 for FM. Between-day TEMs ranged from 0.26-0.90 kg for FM and FFM, with ICCs of 0.995-0.999 for FFM and 0.955-0.998 for FM. Across metrics, DXA demonstrated the best reliability, followed by one MFBIA analyzer and ADP with predicted thoracic gas volume. Overall, laboratory-based body composition devices provide excellent within- and between-day reliability under standardized conditions, though observed differences in error may influence method selection when tracking small body composition changes.