Importance: Anorexia nervosa (AN) is a common psychiatric disorder associated with electrolyte imbalances and impaired kidney function, but their incidence and association with disease severity are unknown.
Objective: To analyze kidney function in patients with AN and its association with body mass index (BMI) and physiologic parameters of disease severity.
Design, setting, and participants: Single-center retrospective case-control study of recently hospitalized patients with a diagnosis of AN according to International Classification of Diseases, Ninth Revision. All patients were aged 9 to 18 years and hospitalized in the general pediatric ward between 2010 and 2019. BMI and estimated glomerular filtration rate (eGFR) were compared with age- and sex-matched controls hospitalized with other diagnoses.
Main outcomes and measures: Impaired kidney function was defined as eGFR less than 90 mL/min/1.73 m2. Association between eGFR, BMI, minimal heart rate, and free triiodothyronine (T3) levels were determined using logistic regression.
Results: A total of 395 patients were included in the study group (81.6% were female; mean [SD] age, 14.6 [2.2] years; median BMI percentile, 12.3 [IQR, 0.9-42.0]). Impaired kidney function was found in 36.8% (146 of 395). Mean (SD) eGFR decreased during hospitalization in the group with kidney function impairment (admission: 83 [10.9] mL/min/1.73 m2; nadir: 79.1 [8.5] mL/min/1.73 m2; latest: 97.7 [15.7] mL/min/1.73 m2; P < .001). Mean (SD) serum creatinine (SCr) to BMI ratio was higher in both anorexia groups compared with controls in impaired kidney function (4.9% [1.0%]), non-impaired kidney function (3.55% [0.84%]); and control groups (2.8 [1.1%]) (P < .001). There was no difference in admission BMI between anorexia groups with and without kidney function impairment. Mean (SD) free T3 levels (3.5 [0.2] pmol/L vs 4.08 [1.2] pmol/L; P < .001) and mean (SD) minimal heart rate (44 [11] beats per min vs 56 [16] beats per min; P < .001) were lower and hospital stay was longer (median, 13 [IQR, 6-21] days vs 8 [IQR, 4-19] days; P = .03) in the impaired kidney function group. The highest correlation was found between minimal heart rate and minimal eGFR (R = 0.53; P < .001).
Conclusions and relevance: Impaired kidney function in patients with AN is common and transiently worsens during hospitalization. SCr values probably underestimate the degree of kidney function impairment in AN. Results of this study found that patients with impaired kidney function had worse anorexia severity parameters unrelated to admission BMI. Therefore, kidney function impairment may be a better marker of anorexia severity.