Extravascular fluids (body cavity fluids other than blood or urine) are examined in various divisions of the clinical laboratory, depending on the nature of the test requested. The extravascular fluids are termed pleural (around the lungs), pericardial (around the heart), peritoneal (around the abdominal and pelvis cavities), synovial (around the joints), and cerebrospinal (around the brain and spinal cord). Each of these fluids is handled in special ways. Synovial fluid and cerebrospinal fluids will be discussed separately. The examination of the other body fluids is discussed in general terms.
CEREBROSPINAL FLUID (CSF)
The usual examination of the cerebrospinal fluid (CSF) specimen includes several observations. Abnormal color, the presence of turbidity, and clot formation are noted. The examination includes cell counts, morphologic examination, chemical analysis, Gram's stain, and cultures. CSF is normally a clear and colorless fluid that circulates in the ventricles of the brain, the subarachnoid spaces, and the spinal cord. It has four main functions: it is a mechanical buffer that prevents trauma, it regulates the volume of the intracranial contents, it is a nutrient medium of the central nervous system (CNS) and it is an excretory channel for metabolic products of the CNS.
Whenever a spinal tap is performed, it is done for serious reasons, for it involves potential harm to the patient. The procedure is done by a physician.
Indications for spinal tap include the following: the diagnosis of meningitis (bacterial, fungal, mycobacterial, or amoebic); the diagnosis of hemorrhage; the diagnosis of neurologic disease such as multiple sclerosis; the diagnosis and evaluation of suspected malignancy such as leukemia, lymphoma, and metastatic carcinoma; and for the introduction of drugs, radiographic contrast media, and anesthetics.
The greatest risk of lumbar puncture involves paralysis or death to tonsillar herniation in patients with increased intracranial pressure. There is also risk of infection form the procedure.
The following laboratory analyses are among those most frequently performed on CSF: gross appearance, cell count and white blood cell differential, protein, glucose, VDRL (for syphilis), Gram's stains, and cultures.
If a disease is present, the CSF may have a cloudy, turbid, bloody appearance, due to the presence of cells or microorganisms. When bacterial infection is present, white blood cells will be increased, protein may be increased, and glucose may be decreased. Bacterial infections most often have a predominance of polynuclear cells, while viral infections most often have a predominance of mononuclear cells.
Synovial fluid is the fluid contained in joints. Synovial membranes line the joints, bursae, and tendon sheaths. The function of the synovial fluid is to lubricate the joint space and transport nutrients to the articular cartilage.
The aspiration and analysis of synovial fluid may be done to determine the cause of joint disease, especially when accompanied by an abnormal accumulation of fluid in the joint (effusion). The joint disease may be crystal-induced, degenerative, inflammatory, or infectious. Morphologic analysis for cells and crystals, together with Gram's stain and culture will help in the differentiation. Aspiration is also done with effusions of unknown origin, and with pain or decreased joint mobility. Effusions of synovial fluid is usually present before aspiration, and therefore it is often possible to aspirate10-20 mL of the fluid for laboratory exam, But sometimes the volume can be quite small, about 1 mL.
In the management of joint disorders, the differential diagnosis is essential so the correct treatment can be instituted. The analysis of synovial fluid can be invaluable in the diagnosis. It can give an immediate diagnosis in some disorders, and provide valuable information concerning other diseases of the joints.
The following laboratory analyses are among the most frequently performed on synovial fluid: gross appearance, viscosity, cell count and white blood cell differential, microscopic examination for crystals, protein, glucose, Gram's stains, and cultures.
SEROUS FLUIDS (PLEURAL, PERICARDIAL, AND PERITONEAL)
Serous fluids are the fluids contained within the closed cavities of the body. These cavities are lined by a contiguous membrane which forms a double layer of mesothelial cells, called the serous membrane. The cavities are the pleural (around the lungs), pericardial (around the heart), and peritoneal (around the abdominal and pelvic organs) cavities. A small amount of serous fluid fills the space between the two layers and serves to lubricate the surfaces of these membranes as they move against each other. The fluids are ultrafiltrates of plasma, which are continuously formed and reabsorbed, leaving only a very small volume within the cavities. An increased volume of any of these fluids is referred to as an effusion.
Effusions may be either transudates or exudates. Transudates occur when plasma filters though the capillary endothelium. They may also be thought of as the result of a systemic disease. Exudates are usually effusions which result from conditions that directly affect the membranes lining the serous cavity.
Pleural fluid is normally present at about 1 to 10 mL, moistening the pleural surfaces. If inflammation occurs, the plasma protein drops, congestive heart failure is present, OR if there is decreased lymphatic drainage, there can be an abnormal accumulation of pleural fluid.
The pericardial space enclosing the heart normally contains about 25 to 50 mL of a clear, strawcolored ultrafiltrate of plasma, called pericardial fluid. When an abnormal accumulation of pericardial fluid occurs, it fills up the space around the heart and can mechanically inhibit the normal action of the heart. In this case, immediate aspiration of the excess fluid is indicated.
Normally, less than 100 mL of the clear strawcolored fluid is present in the peritoneal cavity. An abnormal accumulation of the fluid is indicated by severe abdominal pain and may be caused by a ruptured abdominal organ, hemorrhage resulting from trauma, postoperative complications, or an unknown cause. If this occurs, the excess fluid is aspirated.
The following laboratory analyses are among those most frequently performed on serous fluid: gross appearance, clotting, cell count and white blood cell differential, specific gravity, protein, LDH, glucose, Gram's stains, and cultures.