Urine Protein

All urines contain some protein. The protein excreted by healthy individuals is of the order of 50 to 100 or perhaps 150 mg./24 hr. After considerable muscular exertion the value may be as high as 250 mg. Random specimens will contain under 10 mg./100 ml., although in overnight specimens the level may be as high as 15 to 20 mg./100 ml. Proteinuria is said to be present whenever the urinary protein output is greater than that reflected in these normal values. Not all proteinuria is clinically significant, but persistent abnormal levels of protein in the urine is an indicator of the presence of kidney and urinary tract disease. Thus, the detection of urinary protein and the quantitative assessment of the degree of proteinuria are very important laboratory procedures.

In general, the urinary proteins reflect those present in plasma, along with special proteins of renal tubular origin, and those exuded from the tissues that line the genito-urinary tract. The glomerular filtrate contains small amounts of all plasma protein fractions, but most of the filtered protein is reabsorbed in transit through the kidney tubules. Some of the globulins present in urine are of smaller size than their electrophoretic counterparts in serum. The new techniques of immunoelectrophoresis and antibody-antigen diffusion in agar are rapidly changing our knowledge of urine proteins. The old concept that albumin was the only protein present in any significant amount, even in pathological urines, is no longer valid, and therefore the term albuminuria should no longer be used. Some 40 to 60 per cent of the proteins present in normal urines are mucins (glycoproteins) originating from the linings of the lower  genitourinary tract, and are usually of no clinical significance.

Proteinuria is often a manifestation of primary renal disease, although transient proteinuria may occur with fevers, thyroid disorders, and in heart disease, in the absence of renal disease. Proteinuria may be evident very early in the course of various renal disease states. With such conditions as pyelonephritis, reflecting bacterial infection in the kidney, and acute glomerulonephritis, often associated with recent streptococcal infections, the degree of proteinuria is slight, usually amounting to less than 2 gm. per day. In chronic glomerulonephritis and in the nephrotic syndrome, including lipoid nephrosis, and in some forms of hypertensive vascular disease (nephrosclerosis), protein loss may vary from a few grams to as much as 20 to 30 gm. per day. Proteinuria is encountered in certain other disease entities when and if they give rise to kidney lesions. Among these are lupus erythematosus, amyloidosis, toxemia of pregnancy, septicemia, and certain forms of drug and chemical poisoning. In multiple myeloma, not only does one find a modest increase in protein output, but in 40 to 60 per cent of the cases, the urine contains a group of abnormal proteins referred to as Bence-Jones proteins.

In healthy individuals, transitory elevations in urine protein output are encountered after intense exercise or work, and after exposure to cold. Orthostatic proteinuria is benign condition in which protein excretion is normal when the patient is lying down (prone), but is mildly elevated when the patient walks or stands erect for any period of time. Persons subject to orthostatic proteinuria often are embarrassed during medical examinations for insurance because their urines are found to be positive, when tested for protein after the applicants had been on their feet and active for a good part of the day. On rechecking the urine in the morning after a night's rest, the urine is usually found to be negative for protein.