Quality Control in Microbiology - 1 5%-30% of positive blood cultures represent contamination with skin To keep numbers of contaminants low, perform proper skin Isolation of aerobic, facultative and anaerobic bacteria is best done by tryptic soya casein broth (TSB)

Number of blood cultures: * Number of blood cultures needed to detect bacteremia depends on : Time of collection Type of infection Infecting organism Age of the patient Treatment of patient with antibiotics prior to collection.

Number of bacteria in blood is higher in acute stage of diseases than later stages. Small children have higher numbers at high temperatures. 80% of bacteraemias are detected with the first culture 99% with the first three c ultures.

More than three cultures are not necessary, unless patient has received antibiotics. If time permits, blood cultures are taken at intervals of one hour. If not possible they should be drawn from two different sites.

Blood inoculated in Two Culture Bottles for Aerobic and Anaerobic Cultivation AGE VOLUME Children below 2 yrs2mL in 1 set of bottles Children 2-5 yrs8mL in 2 sets of bottles Children 6-10yrs 12mL in 2 sets of bottles Children 11-15yrs 20mL in 2 sets of bottles Children over 15yrs & adults 30mL in 3 sets of bottles

Cerebrospinal fluid Normal CSF is void of cells and appears colorless and transparent. At least two separate tubes should be collected, each containing 3-10 mL CSF. One tube is forwarded for chemical and microscopic analysis, the other for microbiological investigations.

CSF is cultured under aerobic conditions. Anaerobic culture should be performed if one of the following conditions is suspected : brain abscess, otitis media, mastoiditis, sinusitis, head trauma,or craniotomy. CSF must be transported in a refrigerated box if delay is unavoidable. Presence of fibrin clots in CSF indicates an inflammatory process e.g. tuberculous meningitis.

Total & differential WBC is needed to differentiate bacterial from non- bacterial meningitis. In bact.meningitis-leucocytes count is high. In bact.meningitis,first polymorphs predominate but with recovery these are replaced by lymphocytes.

Polymorphs may be present, along with lymphocytes, in tuberculous meningitis, cerebral abscess,& poliomyelitis. Lymphocytes predominate in viral meningitis, viral encephalitis, neurosyphilis, tuberculous meningitis,& late stages of bacterial infection.

Throat Swab should be obtained before antimicrobial A separate swab is taken for microscopy of diphtheria, candidiasis & Vincent’s If swab is to be processed within 1-2 hours, send to lab. in a sterile test tube. If delayed, send in a transport medium.

@ Throat swab used to : Confirm diagnosis of pharyngitis, diphtheria, epiglottitis, Vincent’s stomatitis and oral candidiasis. Establish focus of infection of scarlet fever, rheumatic fever, and acute glomerulonephritis. Identify carriers of Corynebacteria & Neisseria.

For sore throat, only culture for S. pyogenes & C. diphtheriae. Pneumococci, H. influenzae, S. aureus or coliforms will not cause sore throat and if isolated, should not be reported unless another infection is suspected. In epiglottitis, pharynx should not be swabbed unless a tracheostomy is performed, as this may cause a reflex laryngospasm and may obstruct airway.

@ Interpretation Final diagnosis of diphtheria is obtained after isolation of toxigenic C. diphtheriae from patient. Isolation of S. pyogenes establishes diagnosis of sore throat; but organism should be abundant in culture, since little growth suggests a carrier state.