Background Severity status of rheumatoid arthritis (RA) patient is more extensive characteristic than disease severity as it includes concomitant diseases burden (total comorbidity) as well as decrease of organ functional reserves which steady decline with age.
Objectives To develop Functional Reserves Decline Index (FRDI) including assessment of 5 key organ systems against age: cardiovascular system (cardiac output), respiratory system (maximum breathing capacity), nervous system (nerve conduction velosity), urinary tract (glomerular filtration rate) and tissue metabolism (cellular water) using of currently available nomograms  and to apply it to the severity status of RA patient.
Methods For the calculation of FRDI we have used nomograms by J.F. Fries  which present the dependence of organ functional reserves decline with age for 5 organ systems in the age range 20-90. The nomograms were approximated by means of 5 linear functions: Y1=-0.64×X+77.98 (R2=1), where Y1-5– reserve capacity of the mentioned systems, X – age, years, R2 – coefficient of determination; Y2=-0.97×X+100.87 (R2=1); Y3=-X+105.71 (R2=1); Y4=-0.70×X+89.91 (R2=1); Y5=-0.68×X+100.66 (R2=1).
Results The functional reserve capacities of each organ system for the age of 20, 30, 40 etc. to 90 years were calculated by the presented formulas and summarized for each range of age and the sum was divided by 5. Then a new summarized linear function of the mentioned parameters was plotted against age (see picture below) and approximated by formula: Ysum = 120 – X, where X – age, years. Functional reserve capacity at 20 years was assumed to be 100%. We have suggested to determine FRDI (%) by formula: 100-(120-X) and to indicate the level of FRDI in points as follows: ≤25% – 0 points, 26% to 50% – 1 points, 51% to 75% – 2 points, ≥76% – 3 points. Example: for the age of 75 FRDI is 100-(120-75)=55%, i.e. 2 points.
Conclusions The suggested approach allows taking into account the patient’s age as FRDI (0-3 points) along with disease severity (0-3 points) and total comorbidity assessed by DUSOI (0-3 points) in the severity status of RA patients. The severity status of RA patient can be determined as the sum of these three indices and graded as follows: minimal (1-3 points), moderate (4-6 points), severe (7-8 points) and critical (9 points). The ranges of severity status are refining in the group of 590 RA patients (2625 follow-ups at present).
Fries, J.F.//The Milbank Memorial Fund Quarterly.1983;1:397-419.
Disclosure of Interest None Declared