Introduction: There are two possibilities how to express the status of disproportion between the free radical (FR) concentration and the total antioxidant capacity (AOC): the direct or indirect estimation of FR concentration or the determination of AOC. The aim of this paper is to compare both methods in selected patients, suffering from diseases, associated with this disproportion between FR and AOC values, such as diabetes mellitus and renal insufficiency, treated by haemodialysis. Material and methods: Fresh sera remnants from regular patient control examinations were collected from 30 diabetic subjects and from 30 dialyzed patients. Both groups are age and gender defined. In all samples the following were measured: uric acid (Pliva-Lachema kit, Dimension apparatus), AOC (Randox kit, Cobas Mira) and FR (Sevapharma kit, Vitalab). The obtained results were evaluated statistically by the Student’s test and by regression correlation. The control group comprised 30 samples from blood donors. Results: All results are expressed in mmol/l. FR – control group: 5.71 ± 0.25; D (diabetics): 8.18 ± 1.14; H (dialyzed patients): 7.10 ± 0.36; P < 0.001. AOC – control group: 1.47 ± 0.04; uric acid (UA) – control group: 301 ± 82; D: 1.30 ± 0.60; H: 1.75 ± 0.03; D: 536 ± 142; H: 446 ± 15; P < 0.01. The incidence of important increased values (arithmetic mean ± 2 SD) of estimated parameters in the studied groups: VR (D: 9, H: 7), AOC (D: 7, H: 5), UA (D: 2, H: 6). Discussion: The values of measured parameters found in the control group correspond to physiological values of these parameters. The statistically important elevation of FR concentration in D group, as well as the number of high values of FR found in this group is an implics of oxidative damage which very often occurs in patients with diabetes mellitus. The same result can be expected also in group H. As we found a relatively lower concentration of FR and lower incidence of samples with extremely high values of FR we can assume that it is result of care devoted to the type of dialyzation membrane and the style of treatment of patients with renal diseases. AOC can relate to FR in two different ways. AOC can indicate the decreased values if the antioxidant capacity is exhausted but also higher values, which is interpreted as a form of mobilization of its forces for the defence of the organism. The first alternative is found in D group, the second one in group H. The UA concentration is markedly higher in group D. In group H its concentration is influenced by the high solubility of UA during dialysis. Conclusions: The two methods for determination of antioxidant status are comparable. The simultaneous determination of FR and AOC shows the real situation of the organism concerning its immunity.

        Key words: free radicals, antioxidant capacity, uric acid, diabetes mellitus, haemodialysis.