So the pleural tap is a very important and
key test for patients presenting with pleural
effusions. It’s done by an operator in the
outpatient department. It’s very easy. It’s
very quick. It requires a little bit of local
anaesthetic between the ribs and then insertion
of needle into the fluid and withdrawing 20,
30, 40, 50 ml of pleural fluid to be sent
to the laboratory for investigation.
It’s best done under ultrasound control
to avoid damaging any of the local structures
with the needle insertion. If you drew out
the blood at the effusion and it looks like
there’s blood present, then actually, that’s
quite suggestive that there may be malignancy
or pulmonary embolus. If you drew out the
fluid and it’s visibly turbid i.e. it’s
white and opaque you can’t see through it,
then that would actually suggest potential
empyema. So after pleural tap, the pleural
sent off for multitude of tests.
When analysing pleural fluid, we often have to consider Light's criteria,
which can be used to desurm between an exudated and transudated diffusion.
If any of the following criteria are met, the fluid is considerd to be an exudate.
The pleural fluid protein over serum protein ratio is greater than 0.5,
the pleural fluid LDH over serum LDH is greather than 0.6,
or the pleural fluid LDH is greater than two-thirds the upper limit of normal for serum.
The glucose level may be measured
and that’s matched with the blood glucose
because low glucose suggests infection of
the pleural space.
The fluid is sent off for culture and microscopy
including for acid-fast bacilli mycobacteria
to identify the presence of pleural infection,
although unfortunately, these tests aren’t
particularly sensitive. In fact, for tuberculosis,
it’s actually quite rare to identify the
organism in patients with pleural tuberculosis.
The pH is measured because if that is less
than 7, that is highly suggestive of bacterial
infection. In addition, the fluid is sent
to the histopathologist for cytological examination,
and this is basically to identify cancer cells.
It has a sensitivity which is reasonably good
65% sensitivity for the first pleural tap,
maybe increasing to 75% sensitivity if repeated
with the second pleural tap.
In addition, the cytology identifies the type
of inflammatory cell present. For example,
lots of neutrophils would suggest there’s
pleural infection with bacteria such an empyema.
Lots of lymphocytes might suggest that the
patient has tuberculosis.