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A patient is admitted to the delivery ward in established labour at 36 weeks gestation.

She is hoping for a vaginal delivery and is requesting an epidural. Her blood pressure is consistently high (160/100 mmHg) and has proteinuria.

Which one of the following is the most important test for platelet disorders in this patient?

(Please select 1 option)

Whole blood aggregometry

Platelet count This is the correct answer

Closure time using a platelet function analyser

Template bleeding time

Thromboelastography Incorrect answer selected

Explanation

This patient is likely to have severe pre-eclampsia.

Diagnostic criteria for severe pre-eclampsia is severe hypertension with significant proteinuria, or moderate hypertension diastolic >100 mmHg and proteinuria and two other clinical features or HELLP.

Severe hypertension: systolic BP >160-170 mm Hg or diastolic >110 mm Hg on two occasions.

Significant proteinuria: >1 g/l or 5 g/24 hr or > 3+ on dipstick.

HELLP: haemolysis, elevated liver enzymes, low platelets.

Other features:

Renal: oliguria - <400-500 ml/24 hours (or less than 100 ml over a 4 hour period) or raised creatinine

Pulmonary oedema or respiratory compromise

CNS: headache, visual disturbance, clonus or convulsions

Epigastric pain (or right upper quadrant), liver rupture

Low platelet count: <100 ×109/L

Elevated liver enzymes: ALT or AST > 70 IU.

This question is about tests of platelet disorders and not function.

A normal pregnancy may result in thrombocytopenia. The most important initial test for platelet disorders is the measurement of platelet numbers, size and morphology. This should be performed before any other tests are considered. If the platelet count is <75 ×109/L (there is much interhospital variation in this value), then before performing central neuraxial block, further tests of coagulation should be considered.

Global tests of platelet function: The most widely used tests of platelet function are:

Template bleeding time is subject to laboratory variation, operator technique and therefore poor reproducibility and is invasive.

The platelet function analyser tests whole blood. Blood is passed through a cartridge containing a narrowing. The channel is coated with agonists that trigger platelet adhesion. The end-point of the test is occlusion of the channel.

Thromboelastography (TEG) is a method of testing haemostasis and platelet function in the surgical setting. The use of TEG for routine testing for platelet disorders is yet to be evaluated.

Assays specific to platelet function (these tests are used only in specific circumstances):

Light transmission aggregometry (LTA)

Flow cytometry.

Reference:

Harrison P, et al. Guidelines for the laboratory investigation of heritable disorders of platelet function. Br J Haematol. 2011;155:30-44.

Answer Statistics

1

3%

2

64%

3

8%

4

6%

5

22%

Times answered: 265