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Limitation in diagnostic microbiology laboratoreis


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Limitation in diagnostic microbiology laboratoreis

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Limitation in diagnostic microbiology laboratoreis

  1. 1. Limitation of Diagnostic Microbiology Departments What Clinicians Should Know? Dr.T.V.Rao MDMajority of the Diagnostic Microbiology laboratories are utilisedfor Diagnosis ofBacterial, Fungal and Virus Infections,which haverapid solutions if promptly treated. The word Infection stillconfuses many Clinicians and Microbiologists, and they send thespecimens to laboratories for immediate solutions. The malady ofMicrobiology starts with few qualified nurses to collect specimens,every specimen makes the difference in bacterial flora andcontamination continues to hamper the quality of services. Bloodcollection for blood cultures remain the grey area and major lossof specimen value. The doctors and nurses fail to give properinstructions to the patient. I am certain very few laboratories getan ideal sample,rejecting the sample creates conflict between aclinician and laboratory personal. Everyone at the end of the dayaccepts all the specimens and processed as we practice a path ofleast resistance. A microorganism is judged assensitive or resistantaccording to the diameter of the zone of inhibition of from purifiedisolate from cultural growth, which is then correlated statisticallywith the minimal inhibitory concentration (MIC). The degree ofcorrelation depends on both the antibiotic and the species tested;between 71% and 90% of the results of disc diffusion wereconsistent with the Minimum inhibitory concentration, which isnot done routinely in majority of the laboratories as at it costlierand processing of specimen will increases to more than 10
  2. 2. times.The expected error distribution cannot be reliably predictedby regression analysis. Especially those bacteria which areclassified as having intermediate sensitivity in their inhibitionzones, based on their MIC values, have, in fact, intermediatesensitivity in less than 50% of the cases.It is also necessary to usemethods which take the biological and methodological variationsof daily routine into account. In critical cases such as life-threatening infections or apparent failure of the patient torespond to antibiotic therapy, the MICs of selected agents shouldbe determined. The tests are performed under standardizedconditions so that the results are reproducible. The test resultsshould be used to guide antibiotic choice.When a microorganism isisolated from a patient, the microbiology laboratory will oftenperform susceptibility testing. There is often confusion about whatthese results mean and how it can be used by the clinician to guidethe treatment of the patient. The results of antimicrobialsusceptibility testing should be combined with clinical informationand experience when selecting the most appropriate antibioticfor your patient. We and Many in the Developing world receivethe requests without any proper clinical information about thepatients, many do not write whether he is on any Antibiotictreatment. I request all the Clinicians to remember the patientsspecimens are vital samples and not subjects for blind studies.We Microbiologists report as, the "susceptible" category impliesthat isolates are inhibited by the usually achievable concentrationsof antimicrobial agent when the recommended dosage is used forthe site of infection. (CLSI definition) Our reports are based on
  3. 3. peer reviewed major studies please do note that this definitionsays nothing about the chances of clinical success; in factpredicting clinical outcome based on susceptibility testing and theuse of drugs shown to be in the susceptible category is veryimprecise. This imprecision is due to the effect of host responses,site of infection, toxin production by bacteria that is independentof antimicrobial susceptibility, the presence of biofilms as incatheterised patients, drug pharmacodynamics and other factors.The clinicians should note when reported as resistant it means, the"resistant” isolates are not inhibited by the usually achievableconcentrations of the agent with normal dosage schedules, and/orthat demonstrate zone diameters that fall in the range wherespecific microbial resistance mechanisms (e.g. beta-lactamases)are likely, and clinical efficacy of the agent against the isolate hasnot been reliably shown in treatment studies. (CLSI definition).However, with the exception of urinary bladder infections andsome mycobacterial infections, most clinicians avoid the use of a"resistant" category drug to treat infection. Many cliniciansdemand Zone sizes of sensitivity and resistance to differentantibiotics, they wish to choose the antibiotic showing higher sizeof inhibition for an effective consideration. All clinicians should befamiliar, bigger zone do not mean they are choosing the rightchoice of Antibiotic; ( example if I give a zone of Nitrofurantoin30mm and Cephalosporin as 23 mm which are we choosing in aserious patient with established infection) They have to thinkwhich among many classes of antibiotic suits to his present clinicalcondition. Many clinicians do believe laboratories giving a Zone
  4. 4. sizes are doing a great service however they are doing a greatharm to your patient andin fact they are not following the basicprinciples in Diagnostic Microbiology.Factors affecting thereliability of in vitro testing systems include the limitations ininterpreting MIC data, because in vitro test conditions cannotduplicate the host environment. In vitro testing systems do notconsider the pharmacokinetics of the antimicrobial agent or thepost antibiotic effect, whereby microbial growth is suppressedeven when the antibiotic concentration falls below the MIC.Manyclinicians expect some positive results from Microbiology reportsplease do remember humans contain 10 times more normal florathan our cells together in the body If we start testing antibioticsensitivity for all normal isolates, the wards and hospital will beflooded with resistant strain the future of Hospitals will be injeopardy. We all should be familiar, majority of the patientscoming to a teaching hospitals are treated for several days asoutput patients by private practitioner, many prescribe withoutrationalism the antibiotics influenced by commercial interests.Majority have already used several antibiotics including the thirdgeneration cephalosporins. We have no existing facilities toneutralise the antibiotic in the blood or in any specimen andeconomically not feasible to process the sample as in in developedcountries.Many pathogens like chlamydia, mycoplasmas andtuberculosis need defined conditions to process and culture asthey are harmful to laboratory workers. They can be processedonly in upgraded diagnostic and reference laboratories.It isimportant to understand the limitations of antimicrobial
  5. 5. susceptibility testing. It should be recognized that resistancepatterns will change and guidelines will be subject to periodicrevision. The interpretation of an antimicrobial susceptibility testresult on a laboratory report must never be taken in isolation. It isimportant that all factors are taken into account and that it shouldbe remembered that there is no substitute for sound clinicaljudgment. The expectation of clinicians from Microbiologists is amismatch in majority of the Laboratories in developing countries,as we have no Antibiotic policy and few hospitals invest for thechanging needs, lack of trained technicians and apathy among themany Microbiologists. Conflicts widens with time betweenclinicians and microbiologists if proper investment are not doneand committed Microbiologists take over the Profession, howeverthe serious infected patient’s lifeis at risk.Dr.T.V.Rao MDProfessor and HODDepartment of MicrobiologyTravancore Medical College, Kollam KeralaEmail;