At a Glance
Sepsis is defined as proven or suspected infection in the setting of systemic inflammatory response syndrome (SIRS). Criteria used to define SIRS include tachycardia, tachypnea, leukocytosis or leukopenia, and fever or hypothermia. Sepsis may be associated with other findings in addition to the SIRS criteria, including altered mental status, hyperbilirubinemia, metabolic acidosis, and thrombocytopenia.
Malaria presents as an acute febrile illness that is usually characterized by paroxysmal chills and rigors. Any person with a fever who has a history of travel within an area endemic for Plasmodium spp. should be evaluated for malaria, regardless of his or her history of having taken malarial prophylactic medicine. Malaria can be a debilitating and life-threatening condition and must be treated as a medical emergency.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
Blood Stream Infections (BSIs)
At least two sets of blood cultures should be drawn from two different venipuncture sites when a BSI is suspected. The cultures should be drawn before antibiotics are administered in a hospitalized patient with fever (greater than or equal to 38°C), in the presence of leukocytosis or leukopenia. The peripheral white blood cell count may not be elevated in an immunocompromised patient.
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The microbiology laboratory must be notified if certain pathogens are suspected, as some slowly-growing pathogens require extended incubation of blood bottles beyond the usual 5 days. Brucella spp. and Francisella tularensis usually require prolonged incubation. Although Candida spp. and occasionally Fusarium spp. grow adequately in routine blood culture bottles, the lysis centrifugation culture method should be used if other fungi are considered, such as molds or Histoplasma capsulatum. Bartonella spp. can be isolated using lysis centrifugation.
Positive blood cultures must be interpreted in the context of the clinical scenario and the number of sets submitted. Proprionibacterium acnes, Corynebacterium spp., Bacillus spp., and coagulase-negative staphylococci (excluding Staphylococcus lugdunensis) are usually considered contaminants when recovered from blood cultures. However, each of these organisms may be pathogenic when isolated from immunocompromised hosts or in selected circumstances, such as repeat isolation from multiple sets of blood cultures.
Procalcitonin has been reported as a useful biochemical marker in bacterial infections, specifically in sepsis, although evaluations are still in the preliminary phases. A recent meta-analysis reported that procalcitonin did not adequately distinguish sepsis from nonseptic systemic inflammatory conditions.
Some laboratories have instituted rapid molecular probe- or polymerase chain reaction (PCR)-based methods to identify organisms directly from blood culture bottles. Peptide nucleic acid (PNA) fluorescence in situ hybridization (FISH) offers rapid result reporting of organisms within 3 hours of blood cultures’ turning positive, with high sensitivity and specificity. Several real-time PCR methods for identification of a variety of drug-resistant pathogens directly from positive blood cultures are now commercially available, such as identification of methicillin-resistant Staphylococcus aureus. Broad-range bacterial ribosomal DNA (rDNA) gene PCR, followed by sequencing of the 16S or 23S genes, may be useful if the organism is difficult to grow in culture. However, sequencing assays are not available in most laboratories.
Follow-up blood cultures may demonstrate clearance of the organism once appropriate treatment is initiated. Susceptibilities of blood culture isolates that are repeatedly positive should be performed every 3 days, as organisms may acquire resistance to antibiotics within that time.
Malaria
Blood smears that are examined under the light microscope are performed for malarial diagnosis. A thin smear prepared from a small amount of blood is helpful in determining the species of infecting Plasmodium. Thick smears concentrate the red cell layers approximately 40-fold, which is helpful in screening for low levels of parasitemia. Red blood cells are lysed in the preparation of the thick smear; therefore, a larger quantity of blood can be screened for parasites.
Blood for smear preparation may be collected by venipuncture in an EDTA purple-top tube or by finger-stick. Smears are treated as STAT tests. Preferably, the specific Plasmodium spp. is reported. However, occasionally, Plasmodium falciparum is the only species that can be ruled out, and the report is released as such until definitive species identification is performed. The percent parasitemia is also reported. Follow-up specimens should be submitted to monitor therapeutic responses.
Light microscopy is a labor-intensive and time-consuming process that requires expertise. Simple rapid antigen diagnostic tests (RADTs) based primarily on immunochromatographic lateral flow technology are also available for malarial diagnosis. Malarial antigens include Plasmodium lactate dehydrogenase and aldolase, which are positive in all Plasmodium spp., as well as targets specific for Plasmodium falciparum, such as P. falciparum lactate dehydrogenase and histidine-rich protein-2.
Therapeutic responses can be monitored using the malarial RADTs, as Plasmodium lactate dehydrogenase and aldolase antigen levels decrease with treatment. However, RADTs for malaria require a higher threshold for parasitemia than do blood smears evaluated by light microscopy. For instance, the threshold parasitemia for positivity is generally 100 parasites per microliter of blood for RADTs, whereas a thick smear is capable of detecting 5-10 parasites per microliter of blood. Light microscopy remains the standard of care in determining species and parasitemia load.
PCR, which is becoming more widely available for diagnosis of malaria, boasts a low threshold for detection of parasitemia (1-5 parasites/mL of blood, equivalent to <0.0001% of infected red blood cells).
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
BSIs
The method of obtaining blood cultures is important for accurate results. The technique of the blood draw, number of sets drawn, and the timing of blood cultures are all important determinants. Studies of skin cleansing solutions, including alcohol, chlorhexidine, iodine tincture, and povidone-iodine, have been compared in a recent meta-analysis. If the vein must be palpated after skin preparation, a disinfected or sterile glove should be worn. Blood for cultures should preferably not be drawn solely through in-dwelling intravascular catheters, as these lines are frequently colonized with common skin contaminants.
If a catheter-related infection is suspected, at least one blood culture from a peripheral venipuncture should be collected for comparison to the blood drawn from the catheter. Blood drawn from arteries yields the same results as that drawn from veins. In most instances, there is no scientific basis for obtaining separate blood cultures from various ports of triple-lumen catheters. Needles should not be changed after the venipuncture and before inoculation of the blood culture bottles. Such practices increase the risk of fingerstick injury to the person who is handling the needles.
The optimal number of blood cultures varies according to clinical scenarios. One set of blood cultures is never encouraged, as positive growth in the set may indicate either a pathogen or a contaminant, depending on the organism isolated. For instance, growth of coagulase-negative staphylococci from a single blood culture set is likely due to contamination from the skin during the blood draw. If a second set were available for comparison, the likelihood that the second set would be positive for the same contaminant decreases.
Ideally, 10 mL of blood should be injected into each blood culture bottle for adults, with 0.5-5 mL per bottle for infants and children. The yield of blood cultures increases with higher volumes of blood submitted. Pediatric blood culture bottles, which are smaller and often contain specific antibiotic-binding resins, should be used in small children.
The timing of blood cultures drawn in relation to a febrile episode is not significant. There is no difference in yield between blood cultures drawn simultaneously versus hours apart. Blood culture draws on an acutely ill patient may be performed at two separate sites within minutes of each other, provided that antibiotics have not yet been administered.
Malaria
Occasionally, antimalarial treatment can affect the positivity of the RADTs, as parasite antigens persist to varying degrees after therapy.
What Lab Results Are Absolutely Confirmatory?
BSIs
Recovery of Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, Enterobacteriaceae, Haemophilus influenzae
, Pseudomonas aeruginosa, Bacteroides spp., and Candida spp. from blood culture bottles is almost always significant for BSI. The recovery of enterococci and viridans streptococci from certain patient populations may be significant, particularly if they are recovered from multiple sets.
Malaria
A positive blood smear for malaria is absolutely confirmatory.
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
Water-based povidone products must remain on the skin long enough to dry and release the active ingredient, 1% iodine. As opposed to the alcohol-based products, aqueous products can take up to 5 minutes to dry. In addition, iodine and alcohol-based products lack the longer-term sterilization activity of chlorhexidine, which persists on the skin surface. For these reasons, a chlorhexidine-based approach may result in fewer false-positive results and more appropriate therapy.
Although most institutions define one blood culture set from an adult as an aerobic bottle plus an anaerobic bottle, the incidence of bacteremia that is due to anaerobes is low. Regardless, the anaerobic bottles grow facultative anaerobes more often than they do strict anaerobes. To obtain the recommended blood volume draw, a second aerobic bottle should be inoculated if an anaerobic bottle is not available. One study showed higher organism recovery with the aerobic-anaerobic bottle pair than with the aerobic-aerobic bottle pair. Generally, solely aerobic bottles are employed for pediatric patients, unless there is a predisposition to an anaerobic infection.
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