Dengue and the platelet count: what the number does and does not tell you
Why one platelet count means less than three, why the warning signs are things you feel rather than things we measure, and when to stop reading and go to hospital.
In a Matara June, the platelet count is the most anxiously read number we produce. It deserves some context, because a great deal of worry is spent on it and some of that worry is spent in the wrong place.
This article explains a test. It is not medical advice, and it cannot replace the doctor managing your illness. If you are unwell, the instructions in the last section matter more than anything above them.
What a platelet is
Platelets are the smallest cells in blood and they are what starts a clot. A healthy adult carries roughly 150,000 to 450,000 of them in each microlitre, which is the interval printed beside your result.
In dengue the count commonly falls. It often reaches its lowest point somewhere around the fourth to sixth day of the illness and then recovers, frequently faster than it dropped. That shape, down and then up, is why your doctor asks for the test repeatedly rather than once.
One count is a dot. Three counts are a line
This is the part worth taking away. A single platelet count tells you where you are. It cannot tell you which way you are going, and the direction is what your doctor is actually reading.
A count of 90,000 that was 140,000 yesterday means something quite different from a count of 90,000 that was 70,000 yesterday. The same number, two different conversations. That is why a daily count is asked for during the critical phase, and why we keep every one of your previous counts against your record so the trend can be read without hunting for old papers.
It is also why we report the whole full blood count rather than the platelet line alone. The haematocrit, which is the proportion of your blood made up of red cells, is watched alongside it and rising haematocrit with falling platelets is a pattern clinicians take seriously.
The number is not the emergency
Here is the thing that gets lost. Dengue is not managed by the platelet count alone. Plenty of people pass through a low count uneventfully, and people have become seriously unwell with counts that were not alarming.
What matters clinically is the warning signs, and those are things you notice rather than things we measure. Sri Lanka’s own national guidelines and the World Health Organization list them in much the same terms. Go to a hospital, immediately, if you or the patient has:
- severe or persistent abdominal pain
- persistent vomiting, or an inability to keep fluids down
- bleeding: from the gums or nose, in vomit or stool, or unusual bruising
- extreme tiredness, restlessness, irritability, or a sudden change in alertness
- a drop in temperature accompanied by feeling worse rather than better
- passing much less urine than usual
That last pair catches people out. The fever breaking is not automatically recovery. In dengue, the most dangerous phase frequently begins as the temperature settles.
What we can do
We run full blood counts throughout the day and report them the same day, usually within the hour. If your doctor wants a daily count, tell us at the first visit and we will keep the sequence together so each result is issued beside the last.
If a count comes back at a level your doctor needs to know about now, we do not wait for anybody to open a link. We telephone, and we record who we spoke to and when.
The short version
Get the count your doctor asked for, get it repeated as often as they ask, keep the results together, and watch the person rather than the number. If any warning sign above appears, the platelet count stops being the question. Go.