850 Concise Book of Medical Laboratory Technology: Methods and Interpretations

Problem: False positive results in LJ media

Possible causes Solutions

1. Sputum collected in a contaminated Collect the sputum in a clean sterile container

container

2. Contaminated container used for Clean sterile container is to be used during treatment of sputum to avoid contamination

treatment of the sputum

Problem: Turbidity in phosphate buffer

Possible cause Solutions

1. Prolonged storage of phosphate buffer Gently warm phosphate buffer in a water bath at 25–30°C before usage

at 2–8°C

Problem: No liquefaction of mucoid sputum

Possible causes Solutions

1. Mucolytic reagent used is prepared and Freshly prepared mucolytic reagent should be used. If stored at 2–8°C, should be used

stored at 2–8°C for more than 24 hours within 24 hours

The AFB Smear

The sensitivity of AFB smear for specimen from

extrapulmonary sites is lower than from sputa. The lipidcontaining cell walls of mycobacteria have a unique

characteristic in binding carbol fuchsin stain so tightly

that it resists destaining with strong decolorizing agents

such as strong alcohols and strong acids. This “acid-fast”

staining reaction of mycobacteria, along with their unique

beaded and slightly curved shape, is a valuable aid in the

early detection of infection and monitoring of therapy.

It has been estimated that there must be 10,000 acid-fast

bacilli per milliliter of sputum to be detected by microscopy.

Patients with extensive disease will shed large numbers

of mycobacteria and show a good correlation between

a positive smear and a positive culture. In patients with

minimal or less advanced disease, the correlation of positive

smears to positive cultures may range from 30 to 80%.

Acid-fact stains performed on a weekly basis are

also useful in following the response of patients to drug

therapy. After drugs are started, cultures will become

negative before smears, indicating that the bacilli are

injured sufficiently to prevent replication but not to the

point of preventing binding of the stain. With continued

drug treatment, more organisms are killed and fewer shed,