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. The most important,
perhaps being albumin content because
that dictates whether the patient has a transudate
or an exudate. Less than 30 grams per litre
equals a transudate. Greater than 30 grams per litre
means they have an exudate. In addition
to lactate dehydrogenase, enzyme levels are
measured as these are raised in patients with
exudates. 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.