23. Creatinine - common data for the parameter, principles of the analytical methods, interference, indications for investigation, patient preparation, specimen, reference ranges, interpretation of results.

Creatinine - common data for the parameter, principles of the analytical methods, source of errors and interference, indications for investigation, patient preparation, specimen, reference ranges, result’s interpretation

Creatinine is a chemical waste molecule that is generated from muscle metabolism. Creatinine is produced from creatine. Approximately 2% of the body's creatine is converted to creatinine every day. Creatinine is transported through the bloodstream to the kidneys. The kidneys filter out most of the creatinine and dispose of it in the urine. Its plasma levels are proportional to the muscle mass of the individuals. It is entirely removed by glomerular filtration. It is excreted through the kidneys and changes in creatinine are an early indicator of kidney disease as well as being seen in severe muscle damage or wasting diseases or with many medications such as antibiotics.

Principles of the analytical methods:

1. colorimetric methods – creatinine reacts with picric acid in alkaline solution to form an orange-yellow compound (chromogen)
2. Express reagent strips are also used
3. enzymatic methods with UV detection have high specificity
4. Methods with ion-selective electrodes

Sources of error:

• Positive false readings due to pyruvate acetoacetate, glucose, uric acid, alpha-ketoacids, cephalosporines
• negative false readings if elevated serum levels of bilirubin and hemoglobin – they destroy the chromogen.

Indications for analysis: Indicator of renal function. Used to determine damage to nephrons in late stage kidney damage. (not suitable for early stage kidney damage)

Preparation of the patient and specimen: Blood is obtained under standard Conditions. Specimen: serum without hemolysis, urine.

Reference ranges: Male – 60 - 110 μmol/l Female – 44 - 96 μmol/l

Elevated levels: Due to decreased glomerular filtration by more than 50%; Any condition that impairs the function of the kidneys will probably raise the creatinine level in the blood. Certain drugs can sometimes cause abnormally elevated creatinine levels. Serum creatinine can also transiently rise after ingestion of large amount of dietary meat.

Low values Low blood creatinine levels can mean lower muscle mass caused by a disease, such as muscular dystrophy, or by aging. Low levels can also mean some types of severe liver disease or a diet very low in protein. Pregnancy can also cause low blood creatinine levels