Functional proteinuria

The first determination of total protein in urine was achieved by Cotugno (1736 - 1822) in 1797 by precipitation with nitric acid. He called the detected protein: "albam massam tenerrimo iam coacto ovi albumine persimilem" (dense white substance like the white of a boiled egg). From this, the term "albumin" was later derived (Gressner 2013; Hofmann 2001).

Teissier was the first to describe orthostatic proteinuria in 1899 (Kluthe 2013).

Functional proteinuria is defined as the occurrence of protein excretion in the urine of < 1 g / d, both during the day and at night, under certain conditions (see "Etiology"). If proteinuria does not occur at night, this corresponds to so-called "orthostatic proteinuria" (Herold 2022).

The phenomenon of functional proteinuria can be reproduced at any time in the affected individual (Kluthe 2013).

Predominantly in Anglo-Saxon countries, orthostatic proteinuria is seen as a subtype of physiological proteinuria (Bökenkamp 2020).

Physiologic proteinuria may occur as:

• febrile proteinuria
• orthostatic proteinuria
• stress proteinuria

In order to exclude these physiological proteinurias as much as possible, morning urine should be used when asking about proteinuria (Bökenkamp 2020).

Pathological proteinuria is primarily found due to

• excessive permeability for proteins in the area of the glomerular barrier
• impaired reabsorption of proteins in the proximal tubule (Bökenkamp 2020).

For more details see proteinuria

According to the NHANES- III study, microalbuminuria was found in 8.3% and macroalbuminuria in 1.3% of unselected subjects in the general population. Similar results were also described in the PREVEND study.

In the latter, there were additional statements about a micro-albuminuria of 10 - 20 mg / l (below the valid limits): This was 16.6 % in the total group. It occurred 11.5 % more frequently in hypertensives than in healthy individuals and even 16.4 % more frequently in diabetics (Koziolek 2009).

The following causes may lead to a secondary disturbance of renal function or renal perfusion (Dirksen 2006), triggering functional proteinuria:

• Fever
• physical exertion
• emotional stress
• sleep apnea
• congestive heart failure (see heart failure) (Kasper 2015)
• Heat
• cold (Manski 2020)
• pregnancy (Leps 2003)
• Nephroptosis
• hyperthyroidism
• cardiac insufficiency
• infusions with certain amino acids such as arginine, lysine
• after administration of hydroxyethyl starch (HAES)

(Koziolek 2009)

The vascular endothelium of the capillaries has a functional protective layer in the area of the glomeruli, the so-called ESL = endothelial surface layer. This prevents the passage of plasma proteins and erythrocytes.

Under hemodynamic changes such as pressure, blood flow, etc., the function of this protective layer is disturbed and hematuria or proteinuria occurs (Schurek 2018).

In the case of clinical signs of prerenal proteinuria, the exclusion of pathological proteinuria is particularly important:

• Medical history:

In this case, a previously known kidney disease, diabetes mellitus, arterial hypertension, existing pregnancy should be asked.

• clinical examination:
  • Edema
  • arterial hypertension
• Control examination of the urine sediment
• quantification of proteinuria in 24 h urine
• qualitative examination of the proteins
• further examinations in case of pathological findings (Kribben 2000)

Functional proteinuria can be detected in the laboratory in laboratory animals by e.g. angiotensin infusion (Schurek 2018).

• Albumin:

Normal 8 - 10 mg / 24 h

Functional proteinuria 30 - 300 mg / 24 h

Pathological proteinuria > 300 mg / 24 h

• Albumin / Creatinine:

Normal < 30 mg / g

Functional proteinuria 30 - 300 mg / g

Pathological proteinuria > 300 mg / g (Kasper 2015).

Typical of functional proteinuria are absent changes in urinary sediment (Mader 2013).

• tubular proteinuria
• preglomerular proteinuria
• glomerular proteinuria
• postrenal proteinuria
• Bence-Jones proteinuria (Leps 2003)

see also proteinuria

Since functional proteinuria is not pathological, it does not require therapy. Therefore, the prognosis is good.

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