A 39-year-old patient complained about morning headache, appetite loss, nausea, morning vomiting, periodic nasal hemorrhages. The patient had acute glomerulonephritis at the age of 15. Examination revealed rise of arterial pressure up to 220/130 mm Hg, skin hemorrhages on his arms and legs, pallor of skin and mucous membranes. What biochemical index has the greatest diagnostic importance in this case?