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Rapid microbiology reporting: myth or reality?

SCarcely a month goes by that clinical microbiologists are not urged to buy a system or an instrument promising rapid results. Since the purchase may represent a major portion of the annual capital budget and draw scrutiny by administrators, microbiologists must carefully consider whether the shorter turnaround time will appreciably affect patient care.

It is natural to assume that rapid test results are always preferable to those taking longer to generate. Faster is better--or is it?

"Rapid" is a relative term. A system providing results in four or five hours is certainly more rapid than one requiring 18 to 24 hours. But speed will only be significant if results reach the physician while he or she is still with the patient or thinking about a course of therapy. In my opinion, that is within 30 minutes of the time the laboratory received the specimen.

Results that appear just 10 minutes after the physician has moved on to something else are usually no more useful than results that arrive 10 hours later. Saving a few hours has little practical significance under such circumstances. Laboratory data cannot play a role in patient management until they reach the physician's mind.

Contrary to what one might think, rapid reporting does not really help determine immediae therapeutic response to acute, potentially life-threatening infections. When patients present with what appears to be meningitis, septicemia, acute bacterial endocarditis, diphtheria, epiglottitis, pneumonia, or rapidly progressive muscular infection, they are appropriately cultured, and empiric antimicrobial therapy begins at once. A delay in starting treatment could prove fatal.

Similarly, rapid reporting rarely affects care when the laboratory is asked to identify a pathogen and measure its antibiotic susceptibility in a patient with lower urinary tract infection. Most physicians either initiate empiric treatment at the first visit or wait 24 to 48 hours for urine culture results.

Genuine rapid reporting, making results available within 30 minutes, would be extremely useful in some clinical circumstances. Examples include detection of bacteriuria in outpatients or patients with indwelling catheters who become febrile; intra-operative specimens where results may determine the surgical procedure; and vaginal specimens examined for herpes virus in patients approaching labor, to help the obstetrician decide between vaginal delivery or cesarean section.

There are other situations where early reporting can help--where treatment is promptly initiated, changed, or discontinued on the basis of the microbiology findings. For example, rapid results of throat cultures for group A streptococci or of genital cultures for gonococci and Chlamydia trachomatis in outpatients can guide an early therapeutic decision.

Some rapid methods sacrifice sensitivity or accuracy. Several rapid tests for bacteriuria cannot detect less than 10.sup.5 cfu/ml in urine. Recent studies have indicated, however, that counts of 10.sup.2 cfu/ml are significant in some patients. While direct detection of antigen in cerebrospinal fluid by counterimmunoelectrophoresis takes only 40 minutes, the test is not as sensitive as other methods, such as ELISA, that require several hours.

Figure I lists some of the rapid noncultural procedures widely used in clinical labs for detection of microorganisms. Others not listed, including head-space gas-liquid chromatography (GLC) and DNA probes, remain to be developed for routine clinical lab use. One problem with noncultural methods is that they cannot measure the antibiotic susceptibilities of the microorganisms they detect. Except for beta lactamase tests, there are no rapid methods to determine antibiotic resistance.

Finally, rapid testing is constrained by extra-laboratory factors, which figure prominently in total turnaround time. Regardless of how rapid a test is, the laboratory cannot initiate it until the specimen arrives. Most laboratories still depend on messengers to deliver specimens. Problems with alternatives such as pneumatic tubes and other mechanical transport systems have yet to be worked out.

As I noted, results cannot be used until they are seen or heard by the clinician. Many hospital laboratories use computers to display results instantaneously at remote terminals on the floors. For the noncomputerized laboatory, the telephone remains an effective way to transmit rapid results.

If purchase of rapid system or instrument is contemplated, its cost-benefit ratio must be carefully assessed. A system should not be selected merely because it produces answers in four to six hours or rejected because it requires 18 hours' turnaround.

Rather, ask if it will have a real impact on patient care. Will treatment begin more quickly? Can length of stay be shortened? Do specimen collecting and result reporting methods dilute the effect of rapid results? Are the results sensitive enough to be reported without correlation with a more time-consuming method?
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Author:Ellner, Paul D.
Publication:Medical Laboratory Observer
Date:Jul 1, 1985
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