Unusual case of pernicious anaemia masquerading as thrombotic thrombocytopenic purpura in the setting of multiple normal vitamin B12 deficiency parameters: preventing anchoring and overdiagnosis.

BACKGROUND:Pseudo-thrombotic microangiopathy (pseudo- thrombotic microangiopathy (TMA)) is a rare presentation of B12 deficiency. Overlapping features like elevated LDH/total bilirubin with low haemoglobin/haptoglobin/platelets could deceivingly suggest thrombotic thrombocytopenic purpura (TTP) resulting in avoidable procedures/treatments. CASE PRESENTATION:A 36-year-old female with hypothyroidism initially presented to clinic with fatigue, palpitations, lightheadedness, and dyspnoea over a 3-month duration and was found to have a haemoglobin of 5.7 g/dL. She received two packed red blood cell units in the emergency room and subsequently discharged with outpatient follow-up and empiric oral iron. During her follow-up visit, she was found to have easy bruisability, gum bleeding, and generalized weakness from hemolytic anaemia (mean corpuscular volume (MCV) 90 fL, haptoglobin <8 mg/dL, LDH >4,000 U/L and schistocytosis on CBC) and thrombocytopenia of 52 K/uL. Due to PLASMIC score of 6 and suspicion for TTP, she was transferred to our facility and tr eated with three cycles of plasma exchange and prednisone but were discontinued when ADAMTS13 levels returned normal. While the patient had normal B12 levels, further testing revealed positive intrinsic factor antibodies (IF-Ab) and an elevated MMA level of 1.56 umol/L. Replacement with cobalamin led to normalization of labs and symptoms. CONCLUSIONS:Timely diagnosis of pseudo-TMA was exceptionally challenging due to several overlapping features with TTP including normal B12 and normal MCV. B12 levels may falsely appear normal in pernicious anemia due to IF-Ab interference with chemiluminescent immunoassay. Schistocytes lower the MCV in automated cell counters. Lower reticulocyte index (<2%), presence of immature/large platelets and teardrop cells, elevated MMA and a higher LDH (>2500) are indicative of B12 deficiency.