When should a follow-up sample be taken for a bioassay to confirm results?

• A. 1 week post exposure
• B. 1 day post exposure
• C. 2 weeks post exposure
• D. 1 month post exposure

Answer: (A)

The main idea is to confirm that an initial bioassay result reflects internal contamination rather than just surface contamination or a transient exposure. A follow-up sample is taken to see if there is consistent excretion from an internal burden.

Taking the follow-up about one week after exposure provides a good window: it's long enough for a radionuclide that has entered the body to be mobilized and excreted into urine or feces to produce a measurable signal, but not so long that the activity could drop due to biological clearance or be confounded by new exposures. If the follow-up sample is much sooner, it may not distinguish internal uptake from surface contamination; if it’s much later, the signal could diminish or be harder to interpret.

So, a follow-up around one week post exposure is the best balance for confirming uptake.