Clinical biochemistry

Reference range

S_creatinine men: 62 - 106 mmol/L; women: 44 - 80 mmol/L

S_Urea  2,5 - 8 mmol/L

U_ACR  < 3 g/mol creatinine

U_erythrocytes < 15/µL

U_leukocytes < 20/µL

Chronic kidney disease affects up to 10% of the population; acute kidney injury is often encountered in hospitalised patients, especially in intensive care units. Laboratory tests can reveal both damage to the renal parenchyma and changes in renal function: glomerular filtration rate and tubular function. The basic methods that we use to detect kidney disease are summarized in Figure ... In the first line, we obtain indicative information from chemical urine strip test - every patient with suspected kidney disease should have. It is usually a set of several (often ten) parameters that are measured on individual fields of a plastic strip. After reaction with the relevant substances present in the urine, the detection fields are coloured and read by eye or instrumentation. It is essential to remember that this is a landmark test, to evaluate it in the context of clinical signs and other investigations, and to think about the most common causes of false positive  and false negative results:

• Detection of glucose and blood is based on oxidation - FP results are caused by oxidizing agents (e.g., disinfectants from an improperly treated collection container), FN results are caused by reducing agents (e.g., vitamin C). The principle of glucose determination is the enzymatic breakdown of glucose to form hydrogen peroxide, which oxidizes the detection dye. The principle of blood determination is the pseudoperoxidase activity of heme - it is able to oxidize the detection dye in the presence of H₂O₂. If we realize that heme is detected, it is not surprising to find positive blood in urine even in myoglobinuria (muscle damage) or hemoglobinuria (intravascular hemolysis)
• Detection of protein is based on the acid-base indicator - negatively charged albumin can pull H⁺ out of the acid-base indicator and thus cause discoloration. The buffer in the detection field ensures that the colour of the indicator does not change in dependence on changes in urine pH. However, when the buffer capacity is exceeded at high pH values (  8), discoloration can occur even in the absence of albumin -  FP result. H⁺ only negatively charged proteins (mainly albumin) can be removed from the indicator; other proteins (e.g. myoglobin, Bence-Jones protein) will not be sensitively detected (FN results);

Microscopic examination and quantitative urinalysis further refine the information as needed. Measurement of creatinine and cystatin C in serum is used to estimate glomerular filtration rate.

Figure 10 Basic laboratory methods used todetect kidney disease. RTC - renal tubular cells. Further description intext.

Glomerular filtration rate(GFR) is the amount of plasma filtered by the glomerulus per unit time. It is expressed in ml/s. Decrease in GFR accompanies a variety of diseases and regardless of the cause, it is a summary parameter that gives a good indication of renal function. The GFR data are used, for example, to change the dosage of drugs excreted by the kidneys, for the application of radiographic contrast agents, and for the classification of chronic renal insufficiency. In clinical practice, we do not routinely have methods for accurate measurement of GFR (inulin clearance, iothalamate, radioisotope methods), so we use estimates (estimated GFR, eGFR):

• serum creatinine and calculations derived from it - creatinine is formed in muscles from creatine and phosphate. It is a small molecule that is freely filtered by the glomerulus. Concentration in serum therefore inversely reflects GFR; the largest non-glomerular influence on creatinine levels in serum is muscle mass. Its physiological changes due to age (muscle mass decreases with age), sex (men have more muscle mass than women) are reflected in calculations such as the CKD-EPI equation. However, if there are other factors affecting muscle mass, e.g. patient immobilization, catabolic states, malnutrition, eGFR using serum creatinine will not work well.
• serum cystatin C and calculations derived from it - cystatin C is formed at a relatively constant rate in each nuclear cell. It is a microprotein that is freely filtered by the glomerulus. It is then picked up and degraded in the proximal tubule. Thus, the concentration of cystatin C reflects inversely proportional glomerular filtration rate, and modern equations for estimating glomerular filtration rate have a version for cystatin C (e.g., the CKD-EPI equation). eGFR based on cystatin C is applicable in pregnant children or as a confirmatory estimate of GFR from serum creatinine. Corticotherapy or hyperthyroidism may increase cystatin C production and thus affect the eGFR derived from it. 
• Creatinine clearance - as mentioned above, creatinine freely penetrates the glomerular sieve, but is partially secreted in the tubules - the higher the creatinine concentration in the serum, the greater the secretion in the tubules. We use the formula (urine creatinine concentration x urine volume in 24 hours)/serum creatinine concentration. Secretion into the tubules will increase the concentration in the urine - the higher the creatinine in the serum, the more the clearance overestimates. However, the biggest limit to the use of creatinine clearance is the 24-hour urine collection. This is a task that is demanding for the patient and errors are very common: incomplete collection, inaccurate measurement or just an estimate of the volume of urine collected (should be to the nearest 100 ml).

Tubular functions include acidification ev. alkalinization of urine, resorption and secretion and concentration capacity of the kidneys. A rough reflection of the acidifying capacity of the kidney is the pH of the urine, especially after acid load. The concentrations of ions in the urine or their fractional excretion are used as indicators of resorption and secretion. Concentration capacity of the kidney is monitored by urine osmolality (specific gravity). Water absorption in the distal tubule is mainly controlled by ADH - its production increases with increased plasma osmolality (osmoreceptors in the hypothalamus) and stimulation of carotid baroreceptors by hypotension. The effect of ADH is to open the channels for water - aquaporins in the collecting ducts of the kidneys. If the renal medulla is sufficiently hyperosmolar, passively, along a concentration gradient, water is absorbed => the concentration (specific gravity, osmolality) of urine rises. The hyperosmolality of the marrow is produced by, among other things, the absorption of Na, K and Cl without water in Henle's loop. The disruption of marrow hyperosmolality is the basis for the effect of furosemide, which blocks Na, K, 2 Cl cotransport in the Henle's villus. The marrow is not hyperosmolar and we lose Na, K, Cl in the urine. Na is partially absorbed due to RAAS activation (furosemide is usually given in conditions with secondary hyperaldosteronism), but water is not much absorbed in the collecting ducts even when the aquaporins are open (ADH effect) because the marrow is not hyperosmolar. The result is then a loss of water and a relative conservation of Na.

The indicators of renal damage include mainly proteinuria and haematuria, or the presence of renal tubular cells and granular cylinders in the urine microscopic examination.

Laboratory markers help us to distinguish between renal failurewithout renal parenchymal damage (functional, formerly called prerenal) andwith renal parenchymal damage (renal). In functional failure, we assume full restitution of renal function after adequate treatment - an example is renal failure in dehydration. Typically, we find no evidence of renal damage and tubular function shows effective adaptation - urine specific gravity/osmolality is markedly elevated (water retention in dehydration), urine sodium concentration is low (RAAS activation and adequate tubular response). In renal failure (e.g., damage from toxic drugs, ethylene glycol poisoning, glomerulonephritis), loss of renal function may be permanent. Typically, we find some signs of renal damage - proteinuria (albuminuria), hematuria, renal tubular cells, granular cylinders, and tubular function tends to be impaired. However, there is no specific marker to differentiate and we rely on complex information. The distinction is arbitrary and both renal failures may not initially show signs of renal damage - these only appear in the later course.

To diagnose chronic kidney disease, we use a combination of eGFR and albuminuria - usually expressed as albumin creatinine ratio (ACR). We make the diagnosis if we find eGFR less than 1 ml/s or kidney damage (detected e.g. by ACR or haematuria) and eGFR < 1.5 ml/s at least twice in 3 months. We determine the stage of chronic kidney disease according to the degree of decrease in eGFR and increase in ACR (KDIGO classification).

For the diagnosis of acute renal failure, the CKD-EPI classification is based on changes in serum creatinine (rise in comparison with basal creatinine) and urine production.