An ANM at a sub-centre suspects that a vial of pentavalent vaccine accidentally froze overnight in the ILR. She performs the shake test. The test sedimentation pattern shows the sediment settling slowly and uniformly across the bottom of the vial, matching the control frozen vial. What is the correct action?

• A Use the vial immediately, as uniform settling indicates intact adsorbed antigen
• B Shake vigorously for 5 more minutes and use if the sediment disperses
• C Return the vial to the ILR and repeat the shake test after 24 hours
• D Discard the vial, as this pattern indicates irreversible freezing damage ✓

Explanation

In the shake test, a damaged (frozen) vial shows rapid sedimentation forming a distinct sediment layer similar to the deliberately frozen control, because freezing irreversibly separates the aluminium adjuvant from the liquid medium. An undamaged vial shows slow, diffuse settling without a distinct layer. A vial matching the frozen control must be discarded. Re-freezing, prolonged shaking or delayed retesting does not restore the adsorbed antigen.

Reference: WHO Immunization in Practice, 2015 update ed.

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Written and medically reviewed by the StethoPrep medical team.