Difference between revisions of "Creatinine"

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Structure, properties and matrices

Tautomer forms of creatinine

Creatinine is an organic compound characterized by the chemical formula C₄H₇N₃O with a total molar mass of 113.12 g/mol and a density of 1.09 g/cm³. The solubility of creatinine in water is 90 mg/ml at 20°C. The molecule can tautomerize.^[1] Anhydrous creatinine is a solid substance and generally considered as non-hazardous.^[2]

Creatinine is the anhydride of its precursor molecule creatine (C₄H₉N₃O₂). Creatine is produced by liver, kidneys and the pancreas after which it is transported by the blood to other organs such as muscle and brain, where it is phosphorylated to the high-energy compound phosphocreatine involved in the metabolic process of muscle contraction. Creatinine is a breakdown product of phospocreatine and is normally excreted through the kidneys. Creatinine is usually produced at a fairly constant rate so that its equilibrium concentration is fairly constant in healthy individuals. The equilibrium concentration depends somewhat on diet and muscle mass, and therefore on age, sex, race and body build. Creatinine is commonly measured in blood and urine.^[3]

Clinical significance

Creatinine blood levels often serves as a measure for determining the estimated Glomerular Filtration Rate (eGFR), which is a measure for renal function.^[4][5][6] The GFR in mL/min can be calculated with the following formula: GFR = ( U_X · V̇ ) / P_X. Here, U_X and P_X are the concentrations of substance X in urine and plasma in mg/mL, respectively. V̇ is the urine flow in mL/min. Ideally X is a substance that is not altered, accumulated or resorbed by the kidneys and which has the same concentration in the plasma and glomular filtrate. Using creatinine in this formula, we obtain the creatinine clearance (C_Cr) resembling an eGFR. In the case of renal dysfunction, the creatinine clearance by the kidneys is reduced and therefore the creatinine concentration in the blood rises.^[3]

Inulin is seen as the most reliable method for determining the GFR, measured by an intravenous injection and subsequent sampling of blood and urine. The calculated creatinine clearance turns out to be close to the clearance of inulin. In steady state the rate of urinary creatinine excretion is equal to the metabolic production of creatinine. Pathological conditions, such as hyperthermia, kidney failure or conditions of muscle wasting or damage, disturb the steady state. Overestimation of the GFR can occur at higher concentrations due to hyperexcretion. Consumption of meat, being rich in creatinine, also produces disturbances to the steady state condition.^[3] Although the mentioned method for calculating the eGFR requires measurements of creatinine in urine, plasma concentrations of creatinine compared to normal ranges are also used for an indication of potential kidney failure.

Expected ranges of serum creatinine levels in healthy adults range from 0.6 to 1.2 mg/dL in males and 0.5 to 1.0 mg/dL in females.^{[7] [8] [9]} Creatinine levels in infants and children are generally lower.^[10] On the contrary, creatinine levels in elderly are often higher than average which leads to underestimation of their GFR when using creatinine serum levels as a measure.^[11][12][13] Being a common biomarker for determining the GFR, the creatinine level is also often used for the dosing of some drugs.^[11]

Surveys among general practicioners have indicated a significant desire for creatinine point-of-care tests. Out of 2770 respondents a total of 44% reported that they would use a creatinine point-of-care test.^[14][15]

Lab protocols

Creatinine levels in blood are influenced by multiple anti-coagulants, of which most strongly by citrate. Heparin and EDTA show weaker effects.^[16]

Creatinine is stable for at least 7 days in serum and urine at 4°C. In long term freezing at -20°C and below, serum creatinine is also stable, even after thawing and freezing. If serum and erythrocytes are not separated within 14 hours, the apparent creatinine concentration for some kinetic Jaffe assays is overestimated; this does not happen with enzymatic assays.^[17]

It is important to control the temperature in order to minimize assay variations due to temperature.

State of the art sensing methods

Well established sensing methods of creatinine are chemical (Jaffe), enzymatic (creatininase), isotope-diluted mass spectrometry (IDMS) and high-performance liquid chromatography (HPLC).

The chemical Jaffe reaction is based on creatinine reacting with alkaline picrate, eventually forming an orange-red complex. This yields a measurable color change usable to determine the concentration of creatinine. A large drawback of this method is the low specificity for creatinine. The reaction has many interferrants such as glucose, bilirubin, pyruvate and guanidine. By tuning the reactant concentrations, reading time intervals, temperature and wavelength it is possible to increase the specificity of this method. The reaction is pseudo first order with respect to the picrate until 30 mmol/L. Methods generally use concentrations between 3 and 16 mmol/L, and above 6 mmol/L the color development becomes nonlinear. The reaction is also pseudo first order for hydroxide above 0.5 mmol/L. Above 200 mmol/L blank absorbance increases and above 500 mmol/L the Jaffe complex is degraded faster. For the Jaffe reaction the absorbance is maximal between wavelengths of 490 and 500 nm. Furthermore, temperature control is important since the temperature significantly influences complex formation and absorbance, in the range between 25°C and 37°C. So called compensation factors are often applied in order to account for interferences.^[17]

Primarily, there are three approaches regarding the enzymatic methods. In the first method creatininase amidohydrolase is used to convert creatinine to creatine. The reaction is followed by those of creatine kinase, pyruvate kinase and lactate dehydrogenase. A decrease in absorbance at 340 nm is measured. The initial reaction step allows for prior removement of endogenous creatine and pyruvate. The required incubation time is about 30 minutes. The second approach starts with the conversion of creatinine to creatine using creatinase amidinohydrolase, sarcosine oxidase and peroxidase. The resulting hydrogen peroxide can be detected by a varity of methods. Care must be taken to prevent the inteference of bilirubin and ascorbic acid, e.g. by using bilirubin oxidase and ascorbate oxidase. Alternatively, one can use formaldehyde dehydrogenase instead of peroxidase and measure the reduction of NAD⁺. The last approach employs creatinine deaminase, L-methylhydantoinase, L-carbamoylsarcosine aminohydrolase, arcosine oxidase and peroxidase to, again, produce measurable hydrogen pyroxide.^[17]

There are currently several commercially available biosensors that measure creatinine. Information is available of the i-STAT system made by Abbott, the StatSensor CREAT by Nova Biomedical^[18], and the CardioChek PA by PTS Diagnostics. These sensors all make use of enzymes and electrochemistry to produce a current that linearly correlates with the concentration of creatinine.^[19][20][21]

The numbers in the Table below have been collected per performance aspect. The commercial products combine certain aspects at lowest-performance and other aspects at highest-performance; none of the commercial devices achieve all aspects at highest performance or all aspects at lowest performance.

Note: Creatinine has a molar weight of 113 g/mol. Therefore, a concentration of 1 mg/dL corresponds to a molar concentration of 88.42 μmol/L.

Sensing methods in development

Novel methods are being investigated in order to improve e.g. the assay performance and cost effectiveness. The efforts are mainly oriented toward enzyme-free detection, e.g. by molecularly imprinted polymers (MIPs) or antibodies.

MIPs involve a polymer-based recognition motif that selectively interacts with creatinine, followed by colorimeteric or potentiometric detection.^{[25][26][27][28][29][30]} Picric acid capped silver nanoparticles has been used as selective probes for colorimetric creatinine sensing.^[31] Calix[4]arene thiol functionalized gold nanoparticles have been used to detect creatinine by means of dynamic light scattering. ^[32] Antibodies have been used to make a competitive creatinine immunosensor.^[33]

Potential biomarkers for the future

References