Blood culture test: diagnosis of infection

Blood culture test is a vital diagnostic method to identify bacteria, fungi and other pathogens that have entered the bloodstream. This test is prescribed especially in cases of **high fever, severe infections, or suspected sepsis** (blood infection) and can save patients' lives. In this article, we discuss the method of performing, interpretation of results, applications and limitations of blood culture.

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**1. What is a blood culture test?** Blood culture is a process in which a patient's blood sample is cultured in special environments to confirm the presence of pathogenic microorganisms. This test is done in two ways: - **Aerobic cultivation:** To identify bacteria that need oxygen. - **Anaerobic culture:** To detect anaerobic bacteria such as *clostridium*. **Why is blood culture important?** - Rapid diagnosis of sepsis (a life-threatening condition). - Identification of **pathogens resistant to antibiotics**. - Helping to choose **targeted treatment** for blood infections. **2. When is a blood culture prescribed?** Doctors request a blood culture in the following cases: **High fever without a clear cause** (especially if it is accompanied by chills). ✅ **Symptoms of sepsis:** such as low blood pressure, rapid breathing, or confusion. ✅ **Severe local infections:** such as pneumonia, meningitis, or urinary tract infection. ✅ **before starting strong antibiotics** (to avoid false negative results). ✅ **In patients with weak immune system:** such as those with cancer or HIV.

**3. Blood culture test method ** ### **A) Sampling** - Blood is taken from **two different veins** (usually from each arm) to increase accuracy. - Sample volume: **10-20 ml** for adults (depending on the laboratory protocol). - The sample is placed in **special culture bottles** (containing nutrient medium). ### **b) Cultivation process in the laboratory** 1. **Incubation:** Bottles are placed in automatic devices (such as **BACTEC**) that check the growth of microbes every 10-15 minutes. 2. **Daily Check:** If the microorganism grows, the sample is sent to the microbiology department for **accurate identification**. 3. **Antibiotic sensitivity test (antibiogram):** To determine the most effective drug. **Response time:** - **24-48 hours** for the initial result. - **5-7 days** for final confirmation (in cases of slow-growing infections). **4. Interpretation of results** - **Positive:** The presence of bacteria or fungi in the blood (for example *Staphylococcus aureus* or *E. coli*). - **Negative:** No growth of the microorganism after 5 days (but infection may still be present). **Causes of a false negative result:** Taking antibiotics before the test. Insufficient volume of blood for culture. ❌ Error in sample transportation. **5. The difference between blood culture and other tests** > **NOTE:** CBC may show **increased white blood cells (WBC)**, but does not determine the cause of infection. **6. Blood culture limitations** - **low sensitivity** in some infections (such as endocarditis). - **Relatively high cost** (especially in advanced molecular methods). - **need for correct sampling technique** (preventing sample contamination). **7.