References

Electronic Medicines Compendium (eMC). Co-Trimoxazole 80 mg/400 mg per 5 ml Adult Suspension, Aspen Pharma Trading Limited, Dublin, Ireland. 2018a. https://www.medicines.org.uk/emc/product/6996/smpc (accessed 3 March 2020)

Electronic Medicines Compendium (eMC). Co-Trimoxazole 40 mg/200 mg per 5 ml Paediatric Suspension, Aspen Pharma Trading Limited, Dublin, Ireland. 2018b. https://www.medicines.org.uk/emc/product/6994/smpc (accessed 3 March 2020)

Gigliotti F. Pneumocystis carinii: Has the Name Really Been Changed?. Clinical Infectious Diseases. 2005; 41:(12)1752-1755 https://doi.org/10.1086/498150

British National Formulary for Children 2019-2020.London: Pharmaceutical Press; 2019

The National Institute for Health and Care Excellence. Body surface area in children (image). date unknown. https://bnfc.nice.org.uk/guidance/body-surface-area-in-children-image.html (accessed 12 March 2020)

Calculation Skills

Prophylaxis of pneumocystis jirovecii

02 April 2020
Calculation Skills
02 April 2020
Volume 2 · Issue 4

Check your answers on page 206

Pneumocystis jirovecii is a unicellular yeast-like fungus, which is capable of causing pneumonia in humans, though it is usually only a problem in patients with some degree of immunocompromise, such as those receiving chemotherapy or with HIV/AIDS. The pneumonia caused by Pneumocystis jirovecii is commonly abbreviated to PCP, based on alternative nomenclature for Pneumocystis jirovecii, Pneumocystis carinii (Gigliotti, 2005), hence Pneumocystis carinii pneumonia.

The medication of choice for preventing or treating PCP is the combination antibiotic co-trimoxazole. Co-trimoxazole is a combination antibiotic consisting of a mixture in the ratio 1:5 of trimethoprim and sulfamethoxazole (Electronic Medicines Compendium (eMC), 2018a).

QUESTION 1

The adult liquid formulation contains 480 mg/5 ml (eMC 2018a):

  • What is the amount of trimethoprim per 1 ml of this suspension?
  • What is the concentration of sulfamethoxazole in % weight per volume?

For children, a suggested dose of co-trimoxazole for prophylaxis is 450 mg/m2 twice daily for three days of the week (Joint Formularies Committee, 2019).

QUESTION 2

The back of the BNFC (Joint Formularies Committee, 2019) contains a quick reference table for calculating body surface areas based on the Boyd equation (the table can also be found at The National Institute for Health and Care Excellence (NICE), 2019). Using this table:

  • For a 5.5 kg child calculate the amount of co-trimoxazole required per dose
  • What volume of co-trimoxazole 240 mg/5 ml suspension should be administered per dose (round to one decimal place) (eMC, 2018b)?

In contrast to the prophylactic dose, the treatment dose is 120 mg/kg/day in 2-4 divided doses (Joint Formularies Committee, 2019).

QUESTION 3

For the same 5.5 kg child as above:

  • Calculate the necessary amount of co-trimoxazole to provide treatment using a three times daily dosing regimen.
  • If a smaller volume to administer was preferred, what volume of the adult suspension (480 mg/5ml) would provide the correct dose for this patient (eMC, 2018a)?

From these examples, it can be seen that the way in which doses are expressed for co-trimoxazole products can be confusing, as they may be dosed in terms of either single components or combined drugs, and different dosing regimens may rely on doses in terms of body weight or surface area. However, in spite of this the calculations themselves are relatively straightforward and with a clear plan of what is intended shouldn't prove difficult to most prescribers.