Juvenile sarcoidosis is rare and difficult to diagnose. Although current laboratory tests can guide diagnosis, they are non-specific, and the results should be interpreted in the context of clinical manifestations, imaging findings, and characteristic histological features. Initial work up typically includes basic laboratory tests that often reveal non-specific changes such as elevated ESR and CRP, mildly decreased hemoglobin values, moderate leukopenia, leukocytosis, and/or eosinophilia . As in the present case, abnormalities in liver and kidney function may also be observed, in which case findings from liver or kidney biopsies can aid in histological diagnosis. Rates of hypercalcemia have varied among studies (2–63%), although patients are rarely symptomatic [1, 2]. Typically, patients exhibit increased production of 1,25(OH)2D with low levels of 25-hydroxy-vitamin D and no increases in parathyroid hormone levels [2, 7, 8, 9]. In our patient, 1,25(OH)2D levels were elevated at the time of diagnosis and were more correlated with disease reactivation than serum ACE levels.
Historically, the Kveim–Siltzbach skin test has been used to guide diagnosis, with high sensitivity and moderate specificity [7, 10]. The test is performed by injecting a homogenate of human sarcoidosis lesions intracutaneously, followed by a biopsy 6 weeks later. Unfortunately, this test has a long delay and is no longer widely available for clinical use. Serum lysozyme (LZM) and ACE levels were first highlighted as markers of sarcoidosis in 1973 and 1975, respectively . LZM is secreted from monocytes and polymorphonuclear leukocytes, although the epithelioid cells of the granuloma are the source of serum ACE and LZM when they are both elevated. Moderate increases in serum LZM activity occur in patients with conditions such as acute bacterial infections, leukemoid reactions, megaloblastic anemia, and increased granulocytic turnover. Because serum LZM is less specific for sarcoidosis than serum ACE, the diagnostic value of the test may be limited . However, serum LZM seems suitable for disease monitoring in proven cases. In addition, serum LZM levels demonstrate a significant tendency to increase as the number of organs involved increases. Therefore, the LZM level is regarded as a prognostic indicator rather than a diagnostic tool [11, 12, 13].
The serum ACE level is commonly used as a diagnostic biomarker for the diagnosis of sarcoidosis. ACE is a dipeptidyl-carboxy-peptidase first described as an indicator of diagnosis and response to treatment by Jack Lieberman in 1976. Early studies by Lieberman demonstrated that serum ACE levels are elevated in 90% of patients with sarcoidosis. In some cases, increased serum ACE levels can also predict subsequent changes in clinical status . Later studies reported that serum ACE levels are elevated in approximately 30–80% of adult patients with sarcoidosis, with a sensitivity and specificity of approximately 72% and 60%, respectively . Furthermore, many studies of adult patients with sarcoidosis have reported no significant differences in serum ACE levels between patients with active and inactive disease [11, 14, 15]. Although only a few studies have been conducted in children, their results suggest that serum ACE levels are correlated with disease activity in some cases [9, 16, 17, 18]. However, caution is required when interpreting the significance of serum ACE levels, as the reference interval for the test is age-dependent, and healthy young children may exhibit normal values that are 40–50% higher than those for adults [11, 16, 17]. It is also important to note that serum ACE levels may also be increased in patients with pathologies other than sarcoidosis, such as hyperthyroidism, cirrhosis of the liver, diabetes mellitus, Gaucher’s disease, silicosis, and malignancies. Furthermore, other factors may affect the production of ACE, such as genetic factors and the use of ACE inhibitors [11, 17, 19, 20]. Therefore, serum ACE levels are considered supportive rather than definitive of a sarcoidosis diagnosis. In our patient, the serum ACE level was twice the normal value at the time of diagnosis, supporting the diagnosis of sarcoidosis given the patient’s overall presentation and other features of the disease. As expected, serum ACE levels decreased rapidly after initiating treatment; however, at the time of disease reactivation, levels remained within the normal range despite an increase as shown in Fig. 1.
Various studies have assessed the applicability of other markers in adult patients with sarcoidosis, including adenosine deaminase (ADA) activity, serum amyloid A (SAA) levels, and sIL-2r levels [11, 14, 15, 21]. None of these markers has been extensively studied in patients with juvenile sarcoidosis. Only serum ACE and 1,25(OH)2D levels have been routinely measured in children with sarcoidosis, and the diagnostic and prognostic roles of other biomarkers in such patients remain to be fully determined [9, 16, 17, 18]. ADA is an enzyme involved in purine catabolism that is produced by mononuclear cells and lymphocytes. ADA is widely distributed in human tissues, and the soluble form gives rise to elevated serum levels. Previous studies have reported that serum ADA activity is increased in adult patients with sarcoidosis, especially in untreated patients. However, some studies have reported no significant differences in levels of ADA activity between patients with active and inactive disease [14, 22]. SAA is a marker of inflammation, and its relationship with sarcoidosis activity has been explored in some studies. In one study, SAA levels were elevated in patients with active disease, when compared to those in controls and patients with inactive disease, suggesting that SAA can be used as a marker of disease activity . The role of SAA monitoring in childhood sarcoidosis remains unknown, especially since SAA can be elevated in other inflammatory conditions such as autoinflammatory syndromes.
Serum sIL2-r level has been regarded as a promising biological marker of sarcoidosis in adult patients . The α chain of the IL-2 receptor (also known as CD25) is overexpressed by activated and regulatory T-cells and can be secreted from the cell membrane in a soluble form (sIL-2r) during disease activation. Therefore, sIL-2r is considered a surrogate marker for T-cell activation in patients with conditions such as rheumatoid arthritis, systemic lupus erythematosus, IgG4-related disease, and sarcoidosis. However, it remains unclear whether sIL-2r is produced to combat sarcoidosis-associated immune activation or whether it plays an active role in the pathogenesis of the disease . In one study, elevated serum sIL-2r exhibited superior diagnostic sensitivity and specificity when compared to serum ACE in a group of adult patients with suspected sarcoidosis (sIL-2r: sensitivity, 88%; specificity, 85%; ACE: sensitivity, 62%; specificity: 76%) . Serum sIL-2r levels have also been used to diagnose ocular, pulmonary, and neurosarcoidosis in adult patients [4, 23, 24]. In one study comparing sIL-2r and serum ACE as screening markers for sarcoidosis in adult patients with uveitis, elevated sIL-2r was a more effective marker for sarcoidosis than ACE in patients with uveitis, with a specificity of 94% and sensitivity of 98%. In contrast, ACE was associated with a specificity of 99.5% but a sensitivity of only 22% . In our patient, we evaluated sIL-2r levels as part of the workup for suspected macrophage activation syndrome, as she was febrile and exhibited elevated ferritin levels. The extraordinarily high sIL-2r level of 18,589 U/L (normal range: 223–710 U/L) was intriguing but the bone marrow study was negative for hemophagocytosis. Moreover, we had received the results for elevated serum ACE and 1,25(OH)2D levels, and the patient had undergone renal biopsy that revealed diffuse interstitial nephritis with multifocal non-caseating granulomata. Therefore, we interpreted the elevated sIL-2r level as a marker for sarcoidosis presenting with ocular, renal and pulmonary involvement and decided to follow the trend of this test closely. This interpretation was supported by the increased trend of two markers during disease reactivation (sIL-2r and 1,25(OH)2D), the bone marrow study result (negative for hemophagocytosis), and the promising reports of sIL-2 as a disease marker in adult onset sarcoidosis [3, 4, 23, 24].
We monitored sIL-2r levels along with the two other commonly used markers: serum ACE and 1,25(OH)2D levels. Given the short turnaround time for such tests, these three markers were rather useful for determining responses to treatment and for timely decision-making in our patient. Further prospective studies including larger cohorts of children with sarcoidosis are required to fully elucidate the diagnostic and prognostic roles of the markers noted in this review including serum sIL-2r levels. Until then, we propose that a group of markers should be utilized in order to identify the tests that most closely correlate with disease activity in each patient. This may prove difficult due to limitations in test availability, the cumulative cost of testing, and the turnaround time required to obtain results in some centers.