The complex effects of Long-COVID, a syndrome marked by enduring symptoms after COVID-19 infection, with an emphasis on patients' differing degrees of fibro fatigue (FF). Electrolyte disturbances may affect the severity of FF and may be used as a predictive tool for severe FF in Long-COVID patients. The aim is to use the electrolyte levels for prediction of the Long-COVID patients with high FF levels. The electrolyte levels, calcium, and magnesium, as well as albumin and C-reactive protein levels were measured in 120 Long-COVID patients and 60 controls. FF scale was used for scoring the fatigue severity in all subjects. Patients were divided into high-FF (FF score>25) and moderate-FF group (FF score<25). FF is the major effector on the serum biomarker levels. High-FF group had older people, longer disease durations, lower SpO2, higher CRP, and higher peak temperatures than the control group. High-FF group has a significant decrease in serum total and ionized calcium compared with the controls and low-FF group. After controlling the cofounders, the major factor controlling the levels of the measured biomarkers is the FF value (Partial η2 = 0.468). The ROC-AUC analysis showed that the peak body temperature, Low-SpO2, high-CRP, and low electrolytes can predict the high-FF in a patient with Long-COVID with a moderate sensitivity and specificities (61.6-70%). Long-COVID patients have an elevation in FF score. The decline in electrolytes can predict the severity of FF with moderate sensitivities and specificities.