FLUIDS TYPICALLY USED FOR CLINICAL CHEMISTRY TESTS

    Blood is the most common biologic fluid collected for clinical laboratory testing. It is usually drawn from a vein (in the arm) directly into an evacuated tube. Typically a tube will hold about 5 mL of blood – enough to perform many clinical chemistry tests, since automated analyzers require only small amounts (usually from 2 to 100 μL) for a single test. Occasionally, when collection of blood from a vein is difficult, a sample of capillary blood may be collected by pricking the skin and collecting several drops of blood from the  puncture site. An example is the use of heelstick blood for testing of newborns.

    Other biologic fluids (matrices) often used for testing include urine, saliva, cerebrospinal fluid (CSF), amniotic fluid, synovial fluid, pleural fluid, peritoneal fluid and pericardial fluid. These fluids often contain the same biologic analytes of interest – such as glucose and protein – but differ greatly from each other in physical and chemical properties. These differences in fluid characteristics are termed matrix differences.

    Test methods that are designed for determination of an analyte in blood plasma may not be suitable for determination of that same analyte in other fluids (other matrices). When using a test method for analysis of a fluid other than blood plasma or serum, it is important to validate that the method is acceptable for the type of fluid sample being used.

FLUIDS TYPICALLY USED FOR CLINICAL CHEMISTRY TESTS

Blood (whole blood, serum or plasma)

Urine

Cerebrospinal fluid (CSF)

Amniotic fluid

Saliva

Synovial fluid (fluid that is found in joint cavities)

Pleural fluid (from the sac surrounding the lungs)

Pericardial fluid (from the sac surrounding the heart)

Peritoneal fluid (also called ascitic fluid; from the abdomen)

BLOOD

Blood is the most commonly used specimen for testing in the clinical laboratory. Blood consists of two main parts – a fluid portion (called plasma, which contains the dissolved ions and molecules) and a cellular portion (the red blood cells, white blood cells and platelets). Most clinical chemistry analytes are found in the plasma. Part of the preparation of blood for testing these analytes involves removing the cells. This is done by centrifugation of the sample to pack the blood cells in the bottom of the collection tube and allow removal of the liquid portion for testing.

 URINE

Urine is another fluid commonly used for testing in clinical chemistry laboratories. It is especially suitable for tests that evaluate kidney functions, tests that look at waste products that are excreted by the kidneys, and for metabolites that are cleared quickly from the bloodstream and accumulate in the urine, such as drugs of abuse.

Sometimes both serum and urine concentrations of a substance are useful to know in order to evaluate how well the analyte is being excreted – either to ensure that expected excretion is taking place or to determine if unexpected leakage is occurring.

OTHER FLUIDS

Fluids other than blood and urine, like amniotic fluid, synovial fluid, peritoneal fluid, pleural fluid and pericardial fluid, are used in limited clinical settings and are tested for only a few special analytes.

Amniotic fluid is typically used for tests of fetal health. Spinal fluid is used primarily for assessment of patients with symptoms of diseases such as meningitis or multiple sclerosis or patients who may have suffered a cerebrovascular accident. Chemical testing of fluids such as peritoneal fluid, pericardial fluid or pleural fluid is typically done to assess the origin of the fluid – to determine whether it has leaked from blood vessels because of pressure differences (called a transudate, which is relatively low in protein) or because of inflammation or injury (called an exudate, which is relatively high in protein). Saliva is rarely used in clinical laboratory testing, but is recognized as a specimen whose composition reflects the blood plasma levels of many low molecular weight substances such as drugs or alcohol.

Saliva can be collected without the privacy concerns of observed urine collection for drugs of abuse testing — in order to witness the specimen collection and prevent sample adulteration or substitution by the patient. Saliva also has an advantage for hormones like cortisol for pediatric patients, when blood collection is too painful or stressful. 

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