Generality
Blood culture is a microbiological examination aimed at the isolation and identification of any microorganisms present in the blood; for this reason it is commonly associated with the pharmaceutical susceptibility test of detected pathogens (see antibiogram ). All this guarantees a targeted and effective therapeutic intervention, avoiding unnecessary or harmful antibiotic therapies, both for the subject and for the community.
What's this
Blood culture consists of sowing a blood sample, taken by venipuncture, on special media. This analysis is aimed at finding and identifying one or more microorganisms (especially bacteria or yeasts) responsible for an infection, and then determining their sensitivity to antibiotics.
Note
Bloodstream infections are most commonly of bacterial origin ( bacteremia ), but can also be caused by fungi ( fungemia ) or viruses ( viremia ).
The presence of germs in a blood culture (therefore in the blood of the patient under examination) indicates that a septicemia is in progress. If the latter condition is accompanied by an infectious syndrome one speaks of sepsis .
Why do you measure
Blood culture is used to establish the presence of a systemic infection, sometimes life-threatening. Usually, the source of this complication is located in a specific site of the organism; its diffusion in the blood is favored by the impairment of the immune system, as well as by the location and severity of the infectious process.
Blood culture allows:
- Evaluate the presence of bacteria or fungi in the circulatory stream (ie search for the causative agents responsible for the infection in progress);
- Formulate the microbiological diagnosis of sepsis and / or fever of unknown origin;
- Carry out an antibiogram to direct the doctor to prescribe an effective antibiotic treatment.
Normally, more blood samples are collected to make the exam, from different veins of the two arms. In this way it is possible to increase the probability of detecting bacteria and fungi which may be present in small quantities or which may enter the blood stream intermittently. This procedure also ensures that the pathogenic microorganisms detected are not due to sample contamination, but actually represent the cause of the infection.
When is it performed?
Blood culture is indicated by the doctor when signs and symptoms of sepsis appear as:
- Temperature;
- Chills;
- fatigue;
- Shortness of breath;
- Tachycardia;
- High number of white blood cells.
Pathological conditions such as:
- Endocarditis and endovascular infections;
- Acute epiglottitis;
- Bacterial pneumonia;
- Pelvic inflammatory disease
- Ascending pyelonephritis;
- Hematogenous osteomyelitis;
- Bacterial meningitis;
- Intraabdominal abscesses;
- Immunodepressioni;
- Fever of unknown origin;
- Systemic infections.
Associated examinations
Some related tests may be required to support the outcome of blood culture. These include:
- Gram stain : relatively fast test used to detect and coarsely identify bacteria;
- Sensitivity test : allows to verify if certain drugs (antimicrobials) can be useful in treating the infection;
- Differential blood cell count : evaluates the possible increase in leukocytes, indicative of an infection;
- Urine or sputum culture and / or cerebrospinal fluid analysis : may be indicative of a possible source of infection spread in the blood.
Bacteremia
The presence of bacteria in the blood (bacteremia) or fungus (fungemia) is not necessarily associated with symptoms. In some cases, however, it may be accompanied by an abnormal Systemic Inflammatory Response ( SIRS ); in these cases we speak of sepsis, a clinical syndrome characterized by symptoms such as:
- Hyperthermia (fever) or hypothermia;
- Tachycardia;
- Hyperventilation and dyspnoea (wheezing);
- Oliguria (reduced excretion of urine);
- Skin rush;
- thrombocytopenia;
- Alteration of the leukocyte formula (leukopenia or leukocytosis).
All these signs and symptoms are often blurred in children, the elderly and in subjects receiving corticosteroids or non-steroidal anti-inflammatory drugs.
Normal values
The normal test values include the sterility of the culture.
In the case of septicemia, however, negative blood cultures do not mean the absence of bacterial infection.
Altered Blood Culture - Causes
The presence of microorganisms in the blood can be linked to local or generalized infections, favored by conditions that decrease the body's immune defenses (children, the elderly, chemotherapy, use of cortisone, AIDS, cancer, etc.). In all these situations, the blood culture can allow an extremely early diagnosis, sometimes anticipating the appearance of symptoms and pathological signs.
In physiological conditions, the blood is absolutely sterile; contamination by fungi or bacteria can occur from the outside, as happens in catheterized subjects, or through the lymphatic system. In the presence of an infection the pathogens present in the interstitial fluids are in fact absorbed by the lymphatic capillaries and readily attacked and destroyed in the regional lymph nodes. The possible germs escaped from this capture can enter the bloodstream at the level of the junction between the succlavic veins and the jugular vein (where the lymph flows into the bloodstream).
Pathological changes in blood culture
The presence of microorganisms in the blood can be of character:
- Transitory : eg invasive maneuvers and bladder catheterization;
- Intermittent : localized infections, such as urinary tract infections (IVU);
- Continuous : eg endovascular infections.
The repeated finding of the same germ usually indicates that the origin of the infectious process depends on the identified pathogenic microorganism and the clinical context.
The possible causal conditions are:
- Pulmonary infection;
- Endocarditis;
- pyelonephritis;
- Infection resulting from surgery;
- Animal bites or scratches;
- Nosocomial infection (contracted in hospital).
When different germs are isolated within the same blood culture, septicemia may depend on the weakening of the immune system. In fact, immunosuppressed people have a high risk of developing bloodstream infections, because their defenses are less able to cope with the microorganisms that can enter the blood.
In these cases, it will be necessary to identify the specific cause (eg cirrhosis or immune problems) or the focus of infection (digestive: sigmoiditis; cutaneous: various types of wounds).
Even the use of intravenous drugs or catheters, or the presence of surgical drains, can facilitate entry directly into the bloodstream of bacteria and yeasts.
How to measure it
Just like any other blood test, the blood culture is carried out on a simple blood sample taken from a vein. Normally, the operator makes multiple withdrawals (from two to three) at different sites and times; to avoid possible contamination, the patient's skin is previously disinfected in the sampling areas, while to improve the diagnostic accuracy the nurse can carefully monitor the body temperature and proceed with the collection at the most appropriate time.
The collected blood samples are then analyzed in the laboratory after being inoculated into specific culture media, solid or liquid.
To allow adequate growth of microorganisms, the results of the blood culture test are available only after a few days. The doctor - who in the case of severe symptoms may have already undertaken a generalized treatment - is then sent a series of increasingly detailed reports, until the germ is completely identified with an attached list of antibiotics to which it is sensitive.
The delivery of the definitive results normally takes place within three or four days, but in some cases it may be necessary to wait one or more weeks due to the particularly slow growth of some microbial species sought with blood culture.
Preparation
Blood culture does not require any special preparation on the part of the patient.
Interpretation of Results
- Blood cultures are incubated for a few days before giving a negative or positive result. In the latter case, after identification of the organism responsible for the infection, an antimicrobial susceptibility test is performed in order to prescribe the most targeted therapy.
- Pending the results of blood culture, the patient can still be given broad-spectrum antibiotic drugs, that is able to act towards a high number of bacteria. Once the causative agent has been identified, empirical antimicrobial therapy can be replaced by a specific treatment .
