65. Proteinuria

Proteins aren’t usually filtered out, because most normal plasma proteins are too large for the glomerular pores and they are negative, just like the filter surface. The small amount of protein that is filtered are small in size (below 65 kD) and are reabsorbed by proximal tubular cells where they are metabolized.

Normally is only 1-2 g protein filtered per day, but only 40 – 50 mg is excreted. Anything higher than that is proteinuria, which is often a sign of kidney damage. The proteinuria itself may also cause damage.

Proteinuria

Proteinuria can be classified according to quantity of proteins in the urine.

• Microproteinuria typically means microalbuminuria                       (Urine albumin excretion of 30–300 mg/day)
• Macroalbuminuria (Urine albumin excretion > 300 mg/day)
• Nephrotic syndrome:  massive proteinuria (> 3.5 g/24 h)

Proteinuria has three forms, each with different oetiologies:

• Prerenal proteinuria occurs because the plasma contains an abnormal amount of small-size proteins like lightchains/bence-jones proteins, myoglobin and haemoglobin, which the tubular cells don’t have the capacity to reabsorb. Only small proteins (<65 kD) will be lost.
• Glomerular proteinuria occurs because of a problem with the glomerular filtration. It can occur because of diabetes mellitus, nephrosis or hypertension. If the damage is small will only small proteins be filtered, called selective proteinuria. If the damage is large will both large and small proteins be filtered, called non-selective proteinuria.
• Tubular proteinuria occurs due to problems with the tubules. A common scenario is that tubular hypoxia impairs the tubules’ ability to reabsorb proteins. Only small (<65 kD) proteins will be lost.

All types of proteinuria must be taken as a sign of kidney damage, except orthostatic proteinuria, which is physiological. It may occur because of compression of the renal vein while standing.

Many consequences can occur, mostly because of hypoproteinaemia, and their severity therefore depends on the amount of lost protein:

• Retarded growth
• Muscle atrophy
• Anaemia
• Immune deficiency
• Oedema (decreased oncotic pressure)
• Hyperlipidemia