When I was culturing parasites in the field, one number followed me everywhere: 5%. Five percent haematocrit. It sounds like a small, fussy detail, doesn’t it? Yet it quietly decides whether a culture thrives or collapses.
What haematocrit actually means
Haematocrit is simply the proportion of a suspension that is packed red blood cells. In a culture at 5% haematocrit, red cells make up a twentieth of the total volume, and everything else is culture medium. Think of it as a very thin cellular soup, mostly broth with a modest scattering of cells.
Why not more cells?
Plasmodium falciparum lives inside red blood cells, so you might assume that more cells means more parasites. In practice, a denser culture burns through nutrients quickly, acidifies the medium, and leaves you changing it far more often than you would like. In the field, where supplies are limited and the power can be unpredictable, that matters a great deal.
Why not fewer?
Too sparse, and merozoites struggle to find fresh cells to invade after the schizont bursts. Growth slows, and the parasitaemia you worked so hard to establish stalls. Five percent is a compromise: enough cells for efficient reinvasion, enough medium to keep conditions stable between changes.
What 5% looks like at the bench
Here are two culture sizes I used regularly. In both, the volume is the total culture volume, so the cells are part of it, not added on top.
- For a 5 mL culture: 250 µL of packed red cells (infected cells included) with 4.75 mL of complete medium.
- For a 10 mL culture: 500 µL of packed red cells (infected cells included) with 9.5 mL of complete medium.
If you work from a 50% haematocrit stock instead, double the cell volume: 500 µL of stock for a 5 mL culture, or 1 mL for a 10 mL culture. Top up with complete medium to the final volume.
A few habits that helped me
Always start from properly washed, packed cells, after a clean spin with the supernatant and buffy coat removed. Packed cells are viscous, so use a wide-bore or cut tip and pipette slowly. If you find small volumes of packed cells awkward to measure, make a more dilute stock first and work from that. It is easier to be accurate with a larger volume, and consistency from one passage to the next is what keeps a culture healthy.
The takeaway
Five percent is not magic, but it is a reliable starting point that balances parasite growth against nutrient supply. Once you understand why it works, you can adjust it sensibly when conditions call for something different.
Video callout: Want to see this in practice? Watch the walkthrough on the Adwoa Biotech YouTube channel: https://youtu.be/AF1bkbbgf9w?si=_9ICA7np-ECNXp4s
Related Content
Subscribe by Email
Follow Updates Articles from This Blog via Email
No Comments