Normally, only a small amount (milliliters) of fluid are present in the thorax and abdomen. Effusions, defined as an accumulation
of fluid in one or more body cavities result from multiple causes including increased hydrostatic pressure, decreased oncotic
pressure, increased vascular permeability, decreased lymphatic drainage, infection, neoplastic infiltration, or rupture of
organs or structures within the cavity.
Sample collection is done following aseptic preparation of the site. Local anesthetic or sedation may be required depending
on the site of aspiration and the state of the patient.
Abdominocentesis: ultrasound guided aspiration may be necessary if there is only a small amount of fluid present, however
in most cases non-guided aspiration is adequate. To obtain a cytologic sample, a 3-way stopcock is not needed, however to
remove larger volumes of fluid, a needle with attached stopcock is helpful. Typically a 22g or occasionally a 20g needle attached
to a 12-20 ml syringe is used. The mid-ventral region is most commonly sampled to avoid aspiration of the spleen or bladder.
Thoracocentesis is usually performed from the 6th, 7th, or 8th intercostal space, just below the costochondral junction. The
needle is inserted through the middle of the intercostal space to avoid vessels on the caudal edge of the rib. Often, a 22g
needle, 12-20 ml syringe, and 3-way stopcock is used for aspiration. Alternatively, a needle may be attached to extension
tubing and then to the syringe or stopcock. The patient should be checked frequently for the first few hours following aspiration
to assess respiratory and cardiac function. A thoracic radiograph is indicated if the patient's respiration worsens, to evaluate
for the presence of pneumothorax, particularly tension pneumothorax.
The fluid should be placed in an EDTA tube. If any biochemical analysis is anticipated, it is best to also save some of the
fluid in clot tube. Fluid for culture should be placed in a sterile container, culturette or inoculation broth. Fluid sample
can be sent to a reference laboratory for further processing and interpretation or can be analyzed in-house.
Fluid analysis should include gross evaluation of color and turbidity, nucleated cell count (NCC), determination of total
solids, and cytologic evaluation. The first is to examine the fluid grossly. The appearance and clarity can provide clues
as to the diagnosis and also guide slide preparation and additional sample analysis. Normal fluid and transudates are clear
and colorless. Increased cellularity due to either inflammatory or neoplastic cells produce a cloudy white or opaque color.
Fresh blood or hemorrhage will induce a red-pink coloration; icterus a yellow color, bile will be greenish, and lipid (chyle)
will be milky white. If the specimen is bloody or turbid, it is helpful to centrifuge a small portion (e.g. via microhematocrit)
and repeat the observations on the supernatant.
Next, the total solids (TS) is measured using a refractometer. Total solids is often called total protein however as a refractometer
measures light interference, other solutes will contribute to the final reading. Specific gravity can also be measured using
Cellularity can be quantified using an automated instrument (e.g. a hematology analyzer) or manually using a hemocytometer.
Cellularity can also be estimated from a direct smear, however an estimation of cellularity cannot be determined if the sample
has been sedimented or concentrated by other mechanisms. If there are 'clumps' or other particulate matter in the sample,
a cell count on the fluid portion will typically underestimate the overall cellularity.
Biochemical analysis of the fluid can provide useful information in some cases. Commonly analyzed are triglyceride & cholesterol
(chylous effusion), creatinine (uroabdomen), bilirubin (biliary or gall bladder rupture). Increased fluid LDH may be associated
with septic or neoplastic effusions, while low glucose (< 10 mg/dl) and pH (< 6.9 ) may be associated with septic effusions.