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  Table of Contents  
Year : 2020  |  Volume : 38  |  Issue : 3  |  Page : 469-471

False-positive blood cultures: The need for follow-up

1 Department of Microbiology, SRCC Children's Hospital, Narayana Health, Mumbai, Maharashtra, India
2 Department of Paediatric Infectious Disease, SRCC Children's Hospital, Narayana Health, Mumbai, Maharashtra, India
3 Department of Microbiology, Manipal Hospital, Delhi, India
4 Hematology and Bone Marrow Transplantation, SRCC Children's Hospital, Narayana Health, Mumbai, Maharashtra, India
5 Hemato-Oncology, SRCC Children's Hospital, Narayana Health, Mumbai, Maharashtra, India

Date of Submission31-Aug-2020
Date of Decision28-Aug-2020
Date of Acceptance01-Sep-2020
Date of Web Publication4-Nov-2020

Correspondence Address:
Dr. Suverna Kirolikar
Department of Microbiology, SRCC Children's Hospital, Narayana Health, Mumbai, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijmm.IJMM_20_402

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 ~ Abstract 

The diagnosis of blood-borne infections in immunocompromised patients is a major challenge for the clinical microbiology laboratory. Isolation of bloodborne pathogens in these patients has profound clinical implications, yet is fraught with technical problems, including the presence of unusual and difficult to isolate pathogens. Coupled with this is the problem of false-positive blood culture signals from automated blood culture systems which further delays the definitive diagnosis. Here, we present a case of an 8-year-old boy with Ph +ve acute lymphoblastic leukaemia who has repeated 'false positive' blood cultures and later grew an uncommon organism.

Keywords: Acute lymphoblastic leukaemia, false-positive blood cultures, Mycobacterium abscessus

How to cite this article:
Kirolikar S, Pandrowala A, Joshi S, Misra R, Mushrif S. False-positive blood cultures: The need for follow-up. Indian J Med Microbiol 2020;38:469-71

How to cite this URL:
Kirolikar S, Pandrowala A, Joshi S, Misra R, Mushrif S. False-positive blood cultures: The need for follow-up. Indian J Med Microbiol [serial online] 2020 [cited 2021 Jan 27];38:469-71. Available from:

 ~ Introduction Top

Infections among immunosuppressed patients need inputs from the microbiology laboratory. Blood cultures are the mainstay for the diagnosis. In many cases, the aetiology may not be evident and empiric antibiotic therapy is usually started. When there are false-positive blood cultures signaled by the automated blood culture systems, it becomes frustrating to the clinician who is seeking a diagnosis. Here, we report a similar situation in the case of Ph +ve acute lymphoblastic leukaemia (ALL) who developed febrile neutropenia.

 ~ Case Report Top

An 8-year-old boy with Ph +ve ALL presented with febrile neutropenia post-chemotherapy in August 2019. He had a fever with no focus. The patient at admission was febrile, had pallor with a pulse rate of 110 beats/min and blood pressure of 107/72 mmHg and respiratory rate of 26/min. The chest was bilaterally clear, and clinically all the systems were normal. Laboratory investigations revealed that the following: haemoglobin 7.5 gm, total white blood cell (WBC) count - 7.9 × 103/mm3, differential WBC count-neutrophils 27%, lymphocytes 70%, monocytes 2%, eosinophils 0.2% and basophils 0.3%. Liver function tests and renal function tests were normal, and procalcitonin was 0.31. Blood cultures were sent and viral polymerase chain reaction were negative too. He underwent the chest, abdomen and pelvis computed tomography scan which showed no focus. While on meropenem (120 mg/kg/day) and amikacin (15 mg/kg/day) he persisted to have temperature spikes.

Three blood cultures were done at 24 h intervals, by the PediBact bottle in BACTEC FX 40 system™ (Becton, Dickinson and Company, Franklin Lakes, NJ). The first culture was before the first dose of antibiotics and the subsequent two were done after giving intravenous cefoperazone-sulbactam (150 mg/kg/day). The first blood culture signaled positive after 24 hours. A Gram stain of the blood smear revealed no organisms. Subculture was done on 5% sheep blood agar and MacConkey agar and incubated at 35°C. After 48 h, the second and third blood cultures also signaled positive. Gram-stained smears showed no organism. Subcultures were done on 5% sheep blood agar and MacConkey agar and incubated at 35°C. There was no growth in any of the plates for the 5 days.

In view of no focus and fever for More than 7 days, the Peripherally inserted Central line (PICC) line was removed and the tip was sent for culture.

Diagnostic dilemma in microbiology

  1. The positive signal by the instrument could be due to overloading the bottle and hence giving a false-positive signal. However, false positives due to overloading in three bottles were unlikely
  2. False-positive signals also occur in case of high WBC count, which was unlikely in this case as the child had neutropenia
  3. Or it might have been due to an organism which was not easily stained by the Grams stain.

Differential diagnosis

In a febrile neutropenic child, our differential diagnosis included all opportunistic infections. What we looked for

  1. Gut translocation resulting in Gram-negative sepsis – however, procalcitonin was low and blood cultures were negative
  2. Gram-positive PICC line-associated infection
  3. Fungal - candidaemia or Aspergillus lung infection
  4. Opportunistic infections such as Pneumocystis carinii
  5. Viral infections such as Parvovirus, Cytomegalovirus and EpsteinBarr virus.

Additional investigations

Keeping in mind the above possibilities, additional investigations were done. ZiehlNeelsen (ZN) stain, modified ZN stain for Nocardia and KOH preparation for fungus were done from the BACTEC bottle. ZN stain revealed acid-fast bacilli (AFB) in all the three culture bottles. There were no branching bacilli or fungal elements seen in the other stains.

After 5 days incubation, dry non-haemolytic white colonies were grown on 5% sheep blood agar plate [Figure 1] which were AFB positive by the ZN stain. This was presumptively identified as a rapidly growing Mycobacterium and was further identified by Matrix Assisted Laser Desorption/Ionization time of flight (MALDI TOF) as Mycobacterium abscessus. The PICC line was removed; the culture of the tip also showed the same growth of M. abscessus after 5 days incubation. This isolate could not be sent for antibiotic susceptibility testing due to technical reasons. The child was started empirically on levofloxacin (20 mg/kg/day) and azithromycin (10 mg/kg/day) and became afebrile in 48 h of the start of therapy. He was discharged on both drugs and repeat blood cultures done after 72 h were negative.
Figure 1: Five percent sheep blood agar plate showing white, dry non-hemolytic colonies

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For continuation of chemotherapy, he had a second PICC line inserted in September 2019. Post-chemotherapy, he had febrile neutropenia with pain and swelling of the metacarpal and proximal phalanx of the second finger of the left hand. Levofloxacin was stopped considering the possibility of tenosynovitis. Blood culture on this occasion grew M. abscessus. Repeat chest CT scan did not show lung parenchymal involvement. PICC line was removed and the tip grew M. abscessus. He was started on linezolid (30 mg/kg/day) and amikacin (15 mg/kg/day) to cover M. abscessus awaiting antibiotic sensitivities. Fever and tenosynovitis resolved in 48–72 h of starting the antibiotics and the child was discharged.

Antibiotic susceptibility was done at an outsourced laboratory by the microbroth method (minimum inhibitory concentration [MIC] in brackets);[1] the isolate was susceptible to cefoxitin (16 μg/ml), amikacin (2 μg/ml), clarithromycin (0.25 μg/ml), linezolid (2 μg/ml), imipenem (2 μg/ml), tobramycin (4 μg/ml) and tigecycline (0.5 μg/ml); intermediate susceptibility to ceftriaxone (32 μg/ml) and cefepime (16 μg/ml) and resistant to ciprofloxacin (4 μg/ml), moxifloxacin (4 μg/ml), cotrimoxazole (≥64 μg/ml), doxycycline (≥16 μg/ml), minocycline (≥8 μg/ml) and amoxicillin-clavulanic acid (≥64 μg/ml).

In view of the antibiotic susceptibilities, the treatment was changed to azithromycin (10 mg/kg/dose) and clofazimine (100 mg capsules on alternate days) till December 2019. During inpatient stay for chemotherapy, amikacin was used. The child did not have any repeat bacteraemic episodes after that till date and is doing well.

 ~ Learning Pearls Top

M. abscessus is usually found in water, soil and dust and has been associated with infections after cosmetic and plastic surgery, invasive medical procedures employing contaminated equipment or material, and accidental injury where the wound is contaminated by soil. Immunocompromised state, the presence of an intravascular catheter, transplantation and cancer are also associated with M. abscessus group infections.[2],[3] Isolates are usually resistant to multiple antimicrobials and pose treatment challenges. Our patient probably developed the infection through water contamination.

Microbiological perspectives

Blood cultures play an important role in the diagnostic armamentarium of febrile neutropenia. In automated systems, false-positive signals may occur due to a number of factors. Among the causes of false-positive growth in blood culture systems due to the microorganisms include; (i) slow and fastidious microorganisms, (ii) microorganisms that cannot be cultured, (iii) anaerobic microorganisms, (iv) bacteria more sensitive to environmental conditions such as Streptococcus pneumoniae, (v) culture bottles contaminated with microorganism DNAs and (vi) bacteria transforming into L-form due to antibiotic use.[4]

In the event of a 'false positive' blood culture, additional staining techniques – ZN stain, modified ZN stain and additional culture plates such as chocolate agar, Sabourauds agar would aid in the diagnosis. Prolonged incubation of the plates would reveal the growth of slow-growing organisms.

Turan et al.[4] found a false positivity rate of 0.75% among blood cultures; and false-positive growth signal usually occurred within the first 4 h, and/or in cases of high numbers of leucocytes, neutrophils, or immature granulocytes.

Rapidly growing Mycobacteria can be easily cultured on 5% sheep blood agar after prolonged incubation as was seen in the present case. Identification to species level is important as the antibiotic susceptibility differs between species. MIC by micro broth dilution testing needs to be done for the antibiotics.

Clinical perspectives

Non-tuberculous mycobacteria (NTM) has been reported in adults with haematological malignancies. Incidence is unknown in children with haematological malignancies and therapeutic duration is difficult to determine. In the largest cohort of children with malignancies and NTM infections reported till date from St Jude's children hospital[5]M. abscessus was the second-most common NTM with 85% being catheter-related infections and 7% pulmonary infections. Both patients with pulmonary infection expired. As our patient had a relapse of the infection with the second PICC line, we decided to continue therapy till the end of intensive chemotherapy with the view that his immune system will be better post that with good neutrophil and lymphocyte counts.

Clofazimine has been used for the treatment of M. abscessus infections along with other antibiotics.[6] Since the isolate was fluoroquinolone-resistant and linezolid resulted in myelosuppression, we used a combination of clofazimine as outpatient and amikacin as inpatient with azithromycin for therapy. The child is currently in maintenance chemotherapy cycles, off antibiotics for 4 months and doing well.

Take-home points

  1. Fever without focus in an immunocompromised patient needs a thorough evaluation for cause and keeping the microbiology team well informed of the differentials is essential
  2. Treatment of an immunocompromised child with opportunistic infection should be tailored to degree and duration of immunosuppression
  3. False-positive automated blood cultures must be followed up to enable a definitive microbiological diagnosis.

Declaration of patient consent

The authors certify that they have obtained appropriate patient consent form. In the form the patients parents have given their consent for the clinical information to be reported in the journal. The patient s parent s understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

 ~ References Top

Clinical and Laboratory Standards Institute. Susceptibility testing of Mycobacteria, Nocardia spp.and other aerobic Actinomycetes. 3rd ed. CLSI Standard M24. Wayne PA: Clinical and Laboratory Standards Institute; 2018.  Back to cited text no. 1
Sfeir M, Walsh M, Rosa R, Aragon L, Liu SY, Cleary T, et al. Mycobacterium abscessus complex infections: A retrospective cohort study. Open Forum Infect Dis 2018;5:ofy022.  Back to cited text no. 2
El Helou G, Viola GM, Hachem R, Han XY, Raad II. Rapidly growing mycobacterial bloodstream infections. Lancet Infect Dis 2013;13:166-74.  Back to cited text no. 3
Turan DB, Kuruoglu T, Gumus D, Kalayci F, Serefhanoglu K. Evaluation of factors that may cause false positive growth signals in blood cultures-As the word 'factors' will include both microbial and patients as well as others. Int J Clin Med Microbiol 2018;3:137. [ 4028/2018/137].  Back to cited text no. 4
Apiwattankul N, Flynn PM, Hayden RT, Adderson EE. Infections caused by rapidly growing Mycobacteria spp. in children and adolescents with cancer. J Pediatric Infect Dis Soc 2015;4:104-13.  Back to cited text no. 5
Yang B, Jhun BW, Moon SM, Lee H, Park HY, Jeon K, et al. Clofazimine-containing regimen for the treatment of Mycobacterium abscessus lung disease. Antimicrob Agents Chemother 2017;61:e02052-16.  Back to cited text no. 6


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