Because patients in ICU are commonly plugged with blood catheters, catheter-related blood stream infection (CRBI) can happen and increase the morbidity, the mortality, and the healthcare cost in those patients. Today we talk about the pathogenesis and diagnosis of catheter-related blood stream infections.
The source of catheter-related bloodstream infections are indicated in the figure on the right.
1.Microbes can gain access to the blood stream via contaminated infusates like blood products, but this occurs rarely.
2. Contamination of the internal lumen of vascular catheters can occur through break points in the infusion system, such as catheter hubs. This may be a prominent route of infection for catheters inserted through subcutaneous tunnels.
3.Microbes on the skin can migrate along the subcutaneous tract of an indwelling catheter and eventually reach (and colonize) the intravascular portion of the catheter. This is considered the principal route of infection for percutaneous (non-tunneled) catheters, which include most of the catheters inserted in the ICU.
4.Microorganisms in circulating blood can attach to the intravascular portion of an indwelling catheter. This is considered a secondary seeding of the catheter from a source of septicemia elsewhere, but proliferation of the microbes on the catheter tip could reach the point where the catheter becomes a source of septicemia.
Microbes are not freely moving organisms, and have a tendency to congregate on insert surfaces. When a microbe comes in contact with a surface, it releases adhesive molecules (called adhesins) that firmly attach it to the surface. The microbe then begins to proliferate, and the newly formed cells release polysaccharides that coalesce to form a matrix known as slime (because of its physical properties), which then encases the proliferating microbes. The encasement formed by the polysaccharide maxtrix is called a biofilm. Biofilms are protective barriers that shield microbes from the surrounding environment, and this protected environment allows microbes to thrive and proliferate.
Biofilms on medical devices are problematic because they show a resistance to host defenses and antibiotic therapy. Phagocytic cells are unable to ingest organisms that are embedded in a biofilm, and antibiotic concentrations that eradicate free-living bacteria must be 100 to 1,000 times higher to eradicate bacteria in biofilms.
There are three culture-based approaches to the diagnosis of catheter-related bloodstream infection. The evaluation of suspected CRBI requires one of the three possible decisions for the suspect catheter: 1.remove the catheter and insert a new catheter at a new venipuncture site; 2.replace the catheter over a guide wire using the same venipuncture site; and 3.leave the catheter in place.
Option #1 Remove the catheter and insert a new catheter at a new venipuncture site.
Option #2 Replace the catheter over a guide wire using the same venipuncture site.
Option #3 Leave the catheter in place.
The option #1 is recommended for patients with neutropenia, a prosthetic valve, indwelling pacemaker wires, evidence of severe sepsis or septic shock, or purulent drainage from the catheter insertion site. Otherwise, catheter can be left in place or replace over a guidewire. Option #3 is desirable because most evaluation for CRBI do not confirm the diagnosis (so replacing the catheter is not necessary), and because guidewire exchanges can have adverse effects.
So after the decision of evaluating the diagnosis of CRBI, the following is the culture-based diagnosis procedure. There are three types of culture-based diagnosis procedures fro CRBI, including semiquantitative culture of catheter tip, differential quantitative blood cultures, and differential time to positive culture.
Samples obtaining and Interception
Semiquantitative Culture of Catheter Tip
The standard approach to suspected CRBI is to remove the catheter and culture the tip, as shown below,
1.Before the catheter is removed, swab the skin around the catheter insertion site with an antiseptic solution.
2.Remove the catheter using sterile technique and sever the distal 5 cm (2 inches) of the catheter. Place the severed segment in a sterile culturette tube for transport the the microbiology laboratory, and request a semiquantitative or roll-plate culture. If an antimicrobial-impregnated catheter is removed, inform the lab of such so they can add the appropriate inhibitors to the culture plate.
3.Draw 10 mL of blood from a peripheral vein for a blood culture.
4.The diagnosis of CRBI is confirmed if the same organism is isolated from the catheter tip and the blood culture, and growth from the catheter tip exceed 15 colony forming units (cfu) in 24 hours.
Because the out surface of the catheter is cultured, this method will not detect colonization on the inner (luminal) surface of the catheter (which is the surface involved if microbes are introduced via the hub of the catheter). Nevertheless, semiquantitative catheter tip cultures are considered the “gold standard” method for the diagnosis of CRBI.
Differential Quantitative Blood Cultures
This method is designed for catheters that are left in place, and is based on the expectation that when the catheter is the source of a bloodstream infection, blood withdraw through the catheter will have a higher microbial density than blood obtained from a peripheral vein. This requires a quantitative assessment of microbial density in the blood, where the results are expressed as number of colony forming unites per mL.
1.Obtain specialized Isolator culture tubes from the microbiology laboratory. These tubes contain a substance that lyses cells to release intracellular organisms.
2.Decontaminate the hub of the catheter with an antiseptic solution (use the distal lumen in multilane catheters) and draw 10 mL of blood through the catheter and directly into the Isolator culture tube.
3.Draw 10 mL of blood from a peripheral vein using the Isolator culture tube.
4.Send both specimens to the microbiology lab fro quantitative cultures. The blood will be processed by lying the cells to release microorganisms, separating the cell fragments by centrifugation, and adding broth to the supernatant. This mixture is placed on a culture plate and allowed to incubate for 72 hours. Growth is recorded as the number of colony forming unites per milliliter (cfu/mL).
5.The diagnosis of CRBI is confirmed if the same organism is isolated from the catheter blood sample and the peripheral blood sample, and the colony count in the catheter blood is at least 3 times greater than the colony count in peripheral blood.
Because blood is withdrawn through the lumen of the catheter, this method may not detect microbes on the outer surface of the catheter. However, the diagnostic accuracy of this method is 94% when compared with catheter tip cultures (the gold standard).
Differential Time to Positive Culture
This method is also designed for catheters that remain in place, and is based on the expectation that when a catheter is the source of a bloodstream infection, the blood withdrawn through the catheter will show microbial growth earlier than blood obtained from a peripheral vein. This method uses routine (qualitative) blood cultures; and requires 10 mL of blood drawn through the catheter, and 10 mL of blood from a peripheral vein. The diagnosis of CRBI is confirmed if the same organism is isolated from the catheter blood and peripheral blood, and growth is first detected at least 2 hours earlier in the catheter blood.
This approach is technically easier and less costly than comparing quantitative blood cultures, but he diagnostic accuracy is lower.