The final report of the BDHS 2011 explains the selection of the biomarkers subsample in page 5:

In addition, in a subsample of one-third of the households, all ever-married
men age 15-54 were selected and interviewed for the male survey. In this subsample, a group of
eligible members were selected to participate in testing of the biomarker component, including blood
pressure measurements, anemia, blood glucose testing, and height and weight measurements. Table 1.2
shows which household members were eligible for which biomarker testing.

The subsamples selected for the biomarkers are detailed in the following table of page 6 of the same final report:

Table 1.2 Eligibility for anthropometric measurements and biomarker testing, 2011 Bangladesh DHS