Peripheral blood smear: beyond routine morphology

Introduction

Candida spp. is the third or fourth most common isolate in patients with bloodstream infection. It is the most common genus among yeasts isolated from blood cultures. ¹ The diagnosis of candidaemia from a peripheral blood smear has not been widely reported. This report stresses the importance of a careful examination.

Case report

A 62-year-old postmenopausal woman was diagnosed with ovarian carcinoma with omental metastasis in 2013. After receiving four cycles of neoadjuvant chemotherapy, debulking surgery was planned. Exploratory laparotomy was performed but debulking could not be undertaken owing to advanced disease. Palliative chemotherapy was continued. Initial scans showed reduction in the tumour bulk. Her chemotherapy was continued and in October 2016 she was re-admitted with vomiting and generalised weakness. During this episode, she spiked a fever, investigations for which showed a screen message of an abnormal white blood count scattergram message. According to laboratory protocol, a slide was immediately prepared and stained with Leishmann to perform a manual differential count. This showed neutrophils (88%), lymphocytes (4%) and monocytes (8%). Most of the polymorphonuclear neutrophils showed vacuolisations and also phagocytosed intracytoplasmic yeast cells showing budding at places (Figure 1). Yeast cells were also detected intracellularly in the monocytes as well as extracellularly. Based on these reports and the obvious clinical correlation, intravenous micafungin treatment was initiated, with clearing of the peripheral smear the following day. OPEN IN VIEWER

Simultaneously, aerobic blood cultures were carried out on Bactec 9120 blood culture medium and yeast was detected 24 h later. This was further identified to be Candida Glabrata by automated (Vitek) identification system. Its drug sensitivity was reported as sensitive to Variconazole, Micafungin, Amphotericin B and Flucytosine.

Discussion

Peripheral blood smear has a limited role in diagnosing fungal infection. However, some yeast species detected in peripheral blood smears include Histoplasma capsulatum, Cryptococcus neoformans, Candida spp, Hansenula anomala, Penicillium marneffei and Rhodotorula spp. Candida is the most common cause of fungaemia especially in the setting of ICUs, surgical units, trauma units and neonatal ICUs. Predisposing factors for disseminated candidiasis include immunosuppressive chemotherapy, steroids, indwelling catheters, multiple antibiotic treatment and heart or abdominal surgery.^2,3

Although the diagnostic gold standard for candidaemia is blood culture, yeasts with buds, pseudohyphae or that have been phagocytosed by white blood cells can occasionally be visible microscopically in peripheral blood smears. Budding yeasts on peripheral blood films may be considered as contaminants; however, their intracytoplasmic presence in neutrophils or monocytes is indicative of the pathological nature of the findings. ³ In our case, yeast cells were detected intracellulary as well as extracellulary. A study ⁴ concluded that a possibility of circulating organisms should be considered when abnormal white cell flags are detected. Optimal use of the automated differential requires close correlation between abnormalities flagged by the instrument and a manual review of the peripheral smear. Stringent criteria should be in place in a laboratory so that abnormal flags given by the analyser are manually screened by laboratory professional.

Detection of candidaemia by peripheral blood smear examination requires yeast concentrations of 1–5 × 10⁵ CFU/mL or greater. ⁵ This degree of fungaemia is unusual; therefore, detection of candidaemia by blood smear review is not possible in most cases and is a rare finding. Such detection suggests fulminant infection with uncontrollable complications, usually indicating a poor prognosis.

A number of case reports and small series have described the detection of Candida species (C. albicans, C. glabrata and C. parapsilosis) by review of routinely stained blood smears or by preparation of buffy coat smears.^1–3,6,7 The prevalence of non-albicans Candida species has also increased in recent years. Mortality rates for systemic candidiasis are high, in the range of 50–80%, despite appropriate treatment. ² A favorable outcome requires timely diagnosis. Detection by blood culture requires two to three days of incubation. Molecular methods for the rapid diagnosis of fungaemia are under development but are not yet widely available in clinical laboratories, especially in poor-resource situations. Identification of yeasts in peripheral blood smear therefore allows for a rapid diagnosis and the initiation of timely antifungal treatment. However, good communication is needed to effect this.