Table 1.

Measurement tools to assess physical function [ 26, 28, 55]

Characteristics	Physiological impairment	Functional limitations	Functional disability/self-reported physical functioning
Objective/ subjective	Objective	Objective	Subjective
Examples	Exercise tolerance/capacity tests, cardiopulmonary exercise testing and muscular fitness; maximum strength and muscle endurance	Physical performance tests such as walk tests and functional muscular fitness tests	Self- and/or proxy reports, questionnaires such as SF-36, RAND-36, PROMIS-29 and KDQoL
Basis	Lab-/hospital-based	Mainly lab-/hospital-based	Day-to-day life
Advantages	Gold standard Easy to quantify Less suspect of floor/ceiling effects More useful in comparisons between cultures and geographical environments	Objectively measured Easy to quantify More useful in comparisons between cultures and geographical environments Relatively simple, quick and cost-effective Applicable to large number of individuals Tests represent normal daily activities (walking and sit-to-stand transition)	Patient-focused information Reflects one’s perception of their abilities in their environment Applicable to large number of individuals Easy, cost effective, time efficient and risk free
Disadvantages	May have limited practical utility Requires trained personnel Floor effect for cardiorespiratory tests	No gold standard Potentially suspect of ceiling effects	Subject to external influences Self-reported and therefore lower validity and reliability
Requirements	Trained personnel, expensive equipment and specific analytic skills/knowledge	Familiarization and adherence to protocols	Literacy and ability to understand the language
Practical considerations	Reliant on technical experts Lower feasibility	Less easy to interpret and translate to real-world situation (e.g. walking distance improvement, 1 SPPB point increase)	Choosing the outcome of the self-reported tool necessary to answer the clinical or research question

KDQoL: Kidney Disease Quality of Life.