Association between muscle strength and echogenicity using greyscale ultrasound software: a diagnostic accuracy study in kidney transplant candidates.

Background: Advanced chronic kidney disease disrupts the delicate equilibrium between protein anabolism and catabolism, leading to alterations in muscle quantity, quality, and function. Musculoskeletal ultrasound emerges as a promising assessment tool due to its widespread availability and high reliability.

Aim: To evaluate the efficacy of rectus femoris (RF) echogenicity, measured using greyscale software, in identifying diminished muscle quality and strength in candidates for kidney transplant.

Design: Post-hoc diagnostic accuracy study.

Setting: Outpatients in a multimodal prehabilitation program pre kidney transplantation (KT).

Population: Patients on the waiting list for KT.

Methods: Sensitivity, specificity, likelihood ratios and area under the curve (AUC) for diagnostic efficacy of echogenicity (index test) assessed with the ImageJ software greyscale as a potential marker of quadriceps muscle weakness (reference test) were calculated. Muscle weakness was considered as maximal voluntary isometric contraction of the quadriceps (Q-MVIC) <40% of body weight. Other variables included body composition parameters derived from multifrequency electrical bioimpedance, upper limb muscle strength (handgrip), and RF thickness assessed by ultrasound.

Statistical Tests: Chi-square, t-Student, Pearson correlation coefficients (r), bivariate and multivariate logistic regression models. Statistical significance level ≤0.05.

Results: Of 112 patients (mean age: 63.6, 76% male), 72 (63.7%) exhibited quadriceps weakness, while 80 (70.8%) had some degree of overhydration (extracellular water/total body water ratio >0.390). The echogenicity cut-off point of highest concordance with muscle weakness was 70, boasting a sensitivity of 83%, specificity of 57%, and AUC of 0.671 (CI 95% 0.570-0.772 [P=0.003]). Echogenicity >70 was associated with a 3.4-fold higher risk of muscle weakness (crude OR = 3.4 [CI95% 1.4 to 8.0]), which persisted after adjusting for age, height, weight and RF thickness.

Conclusions: The RF echogenicity exhibits fair validity in identifying muscle weakness among candidates for KT. However, it cannot be endorsed as a standalone diagnostic tool in this population.

Clinical Rehabilitation Impact: Early identification of muscle weakness would advance efforts to mitigate morbidity and mortality through targeted measures.