False-Positive Myeloperoxidase-Antineutrophil Cytoplasmic Antibody in a Patient with Rheumatoid Arthritis

Background

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is an autoimmune disease characterized by small-vessel vasculitis that affects systemic organs, including the lungs and kidney. AAV can present as diffuse lung diseases, such as interstitial pneumonia and alveolar hemorrhage [1]. Therefore, AAV is often included in the differential diagnosis of diffuse lung diseases of unknown etiology [2]. Serum ANCA is an important marker for AAV diagnosis. However, the ANCA level can be nonspecifically elevated in autoimmune diseases such as rheumatoid arthritis (RA) and some infectious diseases [3,4].

Herein, we report a case of pneumocystis pneumonia (PCP) in a patient with RA, which required differentiation from lung disease associated with AAV, owing to false-positive myeloperoxidase (MPO)-ANCA detection. When diffuse pulmonary shadows appear during the course of RA, there are a wide variety of differential diagnoses, including interstitial lung disease associated with RA, drug-induced lung injury, and opportunistic infections. We report this case because it is instructive for diagnosing diffuse lung disease in patients with RA.

Case Report

A 71-year-old woman with a 15-year history of RA treatment was brought to our hospital with a fever and anorexia, which had lasted for 2 days. The patient was admitted to our hospital on the same day because of respiratory failure. Her medical history included RA, hypertension, and chronic kidney disease (CKD). She was initially treated with methotrexate for the RA. Due to renal failure, her medications were changed to prednisolone 5 mg/day and iguratimod 50 mg/day 6 months ago.

She was 158 cm tall and weighed 50 kg. Her vital signs on admission were as follows: temperature, 38.2°C; blood pressure, 162/92 mmHg; pulse, 111 beats/min; and respiratory rate, 24 cycles/min, with an O2 saturation of 91% under oxygen administration at 2 L/min via nasal cannula. Auscultation of the lungs revealed no abnormalities.

Laboratory test results were as follows: white blood cells, 13 200/μL (reference range, 3500-9000/μL), with 82.6% neutrophils; C-reactive protein (CRP), 18.3 mg/dL (reference range, <0.3 mg/dL); blood urea nitrogen, 64 mg/dL (reference range, 8–23 mg/dL); creatinine, 3.54 mg/dL (reference range, 0.5–0.8 mg/dL) (1.32 mg/dL at baseline 1 month prior); and estimated glomerular filtration rate, 10.5 mL/min/1.73 m2 (reference range, >60 mL/min/1.73m2). The Krebs von den Lungen-6 (KL-6) was elevated at 586 U/mL (reference range, <500 U/mL). Urinalysis revealed an occult blood test score of 1+ and a protein level of 1+. The urinary sediment contained 5 to 9 erythrocytes per high-power field, 10 to 19 leukocytes per high-power field, and hyaline cast 1 to 9 per low-power field (Table 1).

Chest radiography and plain computed tomography (CT) revealed diffuse ground-glass opacities in both lungs (Figure 1).

Based on elevated CRP and KL-6 levels and CT findings on admission, acute exacerbation of interstitial pneumonia and respiratory infections, including opportunistic infections, were suspected. In addition, we suspected AAV and performed a serum ANCA test on admission, because of the rapid deterioration of renal function, suggestive of rapidly progressive glomerulonephritis (RPGN). The patient was initially treated with administered intravenous (i.v.) meropenem (1.0 g/day), but the fever persisted. On the fourth day after admission, β-D-glucan and MPO-ANCA levels were founded to be elevated at 99.7 pg/mL (reference range, <20 pg/mL) and 36.0 U/mL (reference range, <3.5 U/mL), respectively. Proteinase 3 (PR3)-ANCA was negative. Laboratory test results suggested PCP and AAV. Bronchoalveolar lavage (BAL) was performed through the right B5 bronchus for a close examination (Table 1). The BAL fluid (BALF) was mildly turbid, and alveolar hemorrhage was ruled out. Grocott staining of the BALF did not confirm the presence of Pneumocystis jirovecii. However, polymerase chain reaction of the BALF was positive for P. jirovecii DNA. Based on these findings, the lung disease was diagnosed, with PCP. After bronchoscopy, the patient was treated with i.v. methylprednisolone pulse therapy (1000 mg/day for 3 days) and oral trimethoprim/sulfamethoxazole (TMP/SMX) (4 g/day), as a tentative diagnosis of PCP and AAV.

A renal biopsy was performed on the seventh day after admission to investigate the cause of acute kidney injury. Pathological test results showed only nephrosclerosis and no evidence of crescent formation or other findings suggestive of RPGN. After admission, the patient continued to receive i.v. supplemental fluids, and renal function improved. Based on her pathological test results and clinical course, the patient was diagnosed with prerenal failure due to dehydration in the background of CKD.

PCP was diagnosed, and TMP/SMX was administered for 2 weeks. Pending the renal biopsy results, prednisolone was carefully tapered, according to the therapy for AAV. The prednisolone dose was immediately reduced after the pathology results were obtained. After these treatments, the chest radiography and CT findings improved (Figure 2). The predniso-lone dose was reduced to 5 mg/day, and the patient was discharged. The clinical course of the patient is shown in Figure 3.

Six months after discharge, her renal function was maintained at baseline. MPO-ANCA showed a decreasing trend but remained positive (10.1–25.0 U/mL).

Discussion

Here, we reported a case of PCP that required differentiation from AAV-associated lung disease. This case is instructive because of the combination of laboratory findings suggestive of AAV, including false-positive MPO-ANCA results and acute kidney injury.

AAV is a form of vasculitis predominantly affecting small blood vessels and systemic organs, including the lungs and kidneys. Microscopic polyangiitis, an AAV, frequently develops with renal involvement. RPGN is the most common form of this disease. It also often causes diffuse lung diseases, such as interstitial pneumonia and alveolar hemorrhage. In this case, microscopic polyangiitis was suspected because of diffuse ground-glass opacities and acute kidney injury on admission.

Serum ANCA is an important marker for AAV diagnosis. ANCA can be measured qualitatively using direct immunofluorescence or quantitatively using enzyme immunoassay. ANCA are classified into p-ANCA and c-ANCA, based on their immunofluorescence staining patterns; the major corresponding antigens used are MPO and proteinase 3 [5,6]. ANCA was previously measured in patients with suspected AAV using a 2-step method: screening with immunofluorescence and enzyme immunoassay for the corresponding antigen [7]. Currently, enzyme immunoassay is the first choice, owing to its good diagnostic accuracy [2]. In diagnosing AAV, MPO-ANCA has a reported sensitivity of 58.1% and specificity of 91.4%. PR3-ANCA has a reported sensitivity of 79.8% to 86.6% and a specificity of 96.8% to 86.6% [8]. It should be noted that ANCA levels are nonspecifically elevated in autoimmune diseases and some infectious diseases. MPO-ANCA level may be nonspecifically elevated, especially in RA [9,10]. In this case, MPO-ANCA level began to decrease after treatment for the infection. Therefore, it has been suggested that infection may also have been involved in the MPO-ANCA elevation.

The coexistence of RA and AAV has also been reported [11,12]. Therefore, when MPO-ANCA is detected in patients with RA, a comprehensive decision should be made regarding the presence of vasculitis. Even if AAV is not present at the time of diagnosis, it can develop later; therefore, patients should be carefully monitored [13].

Renal involvement is highly possible when RA and AAV coexist [13,14]. Our patient’s creatinine level at 1 month before admission was 1.32 mg/dL. Her renal function deteriorated rapidly upon admission, with a creatinine level of 3.54 mg/dL, and RPGN was suspected. A renal biopsy revealed no findings suggestive of vasculitis. The patient continued to experience anorexia due to fever. We concluded that CKD combined with dehydration led to prerenal failure.

Conclusions

When diffuse lung shadows appear during the course of RA, RA-associated interstitial lung diseases, drug-induced lung injury, and opportunistic infections are commonly considered. In addition, RA and AAV can occur together, and numerous differential diseases exist. Serum ANCA is an important marker for diagnosing AAV, but it can also be detected as a false-positive result. Even if MPO-ANCA is positive during a thorough examination for diffuse lung disease or acute kidney injury in patients with RA, it is important to consider the possibility of a false-positive result and perform a thorough and aggressive examination.