TABLE 11

Interpretation of AFB smearsa

No. of AFB seen by carbol fuchsin stain at magnification of ×1,000No. of AFB seen by fluorescent stain at magnification ofb:Interpretation according to:
×250×450CDCWHO
0/smear0/smear0/smearNo AFB seenNo AFB seen
1–2/300 fields1–2/30 fields1–2/70 fieldsDoubtful; repeatcConfirmation requiredd
1–9/100 fields1–9/10 fields2–18/50 fields1+Scanty
1–9/10 fields1–9/field4–36/10 fields2+1+
1–9/field10–90/field4–36/field3+2+
>9/field>90/field>36/field4+3+
  • a See references 75 (CDC) and 177 (WHO). AFB, acid-fast bacilli.

  • b If using a different magnification, interpretations must be adjusted.

  • c Although counts of <3 AFB per 100 fields are not considered positive, a second AFB smear from the same sample may be stained and examined, or another specimen should be requested.

  • d Confirmation by another technician or by preparing, staining, and reading another AFB smear is required.