The majority of investigators who have studied the problem of kidney function, particularly in relation to impaired elimination, accord increasing importance to the estimation of nitrogenous substances in the blood. Though the relation of nitrogen retention in the blood to the pre-uremic and uremic manifestations in the clinical picture of nephritis is not sufficiently understood to allow definite interpretation as to diagnosis and prognosis, there can be no doubt that nitrogen retention is intricately connected with disturbances arising from impaired kidney function.
Among the nitrogenous substances of the blood, urea has received particular attention for a number of reasons which need not be analyzed here. As a result of many careful investigations, the retention of urea in the blood is now generally regarded as a reliable diagnostic sign of faulty kidney function; but, on the other hand, there still remains a disturbing degree of confusion as to what should be
KAST L, WARDELL EL. REMARKS ON THE CONCENTRATION OF UREA IN HUMAN BLOOD. Arch Intern Med (Chic). 1918;XXII(5):581-592. doi:10.1001/archinte.1918.00090160030003