INTRODUCTION

Sarcoidosis is a major cause of uveitis worldwide.¹ An accurate diagnosis of sarcoidosis in uveitis patients has important consequences for managing patients’ care and visual outcomes as well as medicinal options.^2–5 Determining whether an uveitis patient also has sarcoidosis is usually established by combining chest imaging and biochemical measures and then preferably confirmed by biopsy results.^6–9 The lack of a highly sensitive and specific sarcoidosis screening test in uveitis patients poses a major problem in diagnosing, because undetected sarcoidosis can lead to substantial systemic and ocular morbidity.¹ Although serum ACE (sACE) is the most commonly used diagnostic and activity biomarker for sarcoidosis, it has a sensitivity varying from 40% to 83%.^{7 10–13} Lymphopaenia, a result of peripheral T-cell depletion, occurs in up to >50% of sarcoidosis patients and is associated with a chronic course of the disease.¹⁴ However, large studies investigating lymphopaenia’s value in predicting sarcoid uveitis remain scarce.^15–17 This study was conducted to investigate the value of lymphopaenia and elevated sACE, singly or in combination, as diagnostic biomarkers of sarcoid uveitis.

RESULTS

Population characteristics

Nine hundred and ninety-six adult subjects with undifferentiated uveitis underwent measurement-based screening for sarcoidosis. After excluding 125 patients whose CBC and/or serum ACE measurements were missing plus 3 patients treated with ACE inhibitors, 868 patients were analysed. The characteristics of the study population and final diagnosis are reported in table 1. The mean age at uveitis onset was 49.4±18.6 years; 509 patients (58.6%) were female and 689 of the patients (79.3%) were of Caucasian descent.

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Table 1

Basic characteristics of consecutive patients with a first uveitis attack (data are presented as number of cases and percentages (%) unless otherwise specified)

Sarcoid uveitis was diagnosed in 144 subjects. Three hundred and forty-nine patients were classified as indeterminate cases and were included in the other diagnoses group. The mean age at uveitis onset in the sarcoidosis group was 57.3±16.9 years. There was a preponderance of female (58.6%) and white European patients (79.4%), but these characteristics did not differ from the comparison with non-sarcoid uveitis patients (p=0.161 and p=0.663, respectively) (table 2). Panuveitis and bilateral involvement were more common in sarcoid-associated uveitis (51.4% and 77.1%, respectively) compared with patients with non-sarcoid uveitis (27.1% and 54.7%; p<0.001 and p<0.001, respectively) (table 2). Sarcoidosis was definite (biopsy-proven) in 102 subjects (71%) and presumed in 42 subjects (29%). Among those 42 patients, 32 were judged to have sarcoid uveitis on the basis of a positive 18-F-FDG PET/CT scan suggestive of sarcoidosis, 9 had alveolar lymphocytosis, while 1 patient had possible neurosarcoidosis. HCRT was performed in the 144 patients diagnosed with sarcoid uveitis (100%).

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Table 2

Comparison between sarcoid uveitis and non-sarcoid uveitis (data are presented as number of cases and percentages (%) unless otherwise specified)

The median sACE level was 51.4 IU/L (IQR 35.3–72.0). In subjects with sarcoidosis, median sACE was 79.0 IU/L (IQR 54.0–99.5) whereas in subjects without sarcoidosis, median sACE was 47.0 IU/L (IQR 33.1–64.1) (p<0.001). The sACE was elevated in 147 subjects (16.9%) and elevated levels were observed in 66 subjects with sarcoid uveitis (true positives, 45.8%) and in 81 subjects with an alternate diagnosis (11.2%) (false positives, 55.1%) (p<0.001). Older age appeared somewhat associated with elevated sACE although the difference did not reach statistical significance (Wilcoxon–Mann–Whitney test, p=0.058). Subjects were more likely to have elevated sACE if they had intermediate uveitis or panuveitis (χ² test, p=0.007), or granulomatous uveitis (Fisher’s exact test, p<0.001), or if they were of North African or Afro-Caribbean origin (Fisher’s exact test, p=0.012). The most common diagnosis in the false-positive group was idiopathic uveitis (n=36, 44.4%). Other diagnoses in the false-positive group included: HLA-B27-related anterior uveitis (n=5, 6.2%), infectious uveitis (n=21, 25.9%; syphilis, tuberculosis, herpes simplex virus and Lyme’s disease). There were also six cases of birdshot retinochoroidopathy, one Behcet’s disease, one Vogt Koyanagi Harada’s disease and two cases of ocular lymphoma.

In subjects with sarcoid uveitis, the median lymphocyte count was 1.50×10⁹/L (IQR 1.10–1.90) whereas in subjects without sarcoid uveitis, the median lymphocyte count was 2.00×10⁹/L (IQR 1.52–2.54) (p<0.001). Overall, lymphopaenia was observed in 46 subjects (5.3%). It was observed in 22 subjects with sarcoid uveitis (15.3%) (true positives, 47.8%), plus in 24 subjects having an alternate diagnosis (3.3%) (false positives, 52.2%) (p<0.001). Granulomatous uveitis (p=0.006) and older age (p<0.001) were also associated with lymphopaenia. The most common diagnosis in the false-positive group was idiopathic uveitis (n=13, 54.2%). Other diagnoses in the false-positive group included HLA-B27-related anterior uveitis (n=2, 8.3%) and infectious uveitis (n=4, 16.6%; herpes simplex virus and toxoplasmosis). There were also three birdshot retinochoroidopathy and two ocular lymphoma diagnoses.

The number of patients in each category is shown in table 3.

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Table 3

Number of patients in each category

RELIABILITY OF SERUM ACE AND LYMPHOPAENIA IN SYSTEMIC SARCOIDOSIS DIAGNOSING

Using the standard cut-off of 68 IU/L, SE of sACE in the diagnosis of sarcoid uveitis was 45.8%, Sp was 88.8%, PPV was 44.9% and NPV was 89.2% (table 4). AUC was 0.778 (95% CI 0.72 to 0.836, p<0.001) (figure 1). Based on this ROC curve, the optimal cut-off for sACE to maximise both specificity and sensitivity in adult subjects was 61.2 IU/L, resulting in SE of 72.3% and Sp of 72.5%. The sensitivity for lymphopaenia in the diagnosis of sarcoid uveitis was 15.3%, Sp was 96.7%, PPV was 47.8% and NPV was 85.2%. AUC was 0.713 (95% CI, 0.655 to 0.771, p<0.001) (figure 2). Based on this ROC curve, the optimal cut-off for lymphopaenia to maximise both specificity and sensitivity in adult subjects was 1.74 G/L, resulting in a SE of 63.7% and a Sp of 65.3%.

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Table 4

Summary of test characteristics of lymphopaenia, elevated ACE and the combination of both for the diagnosis of sarcoid uveitis (data are presented as percentages (%))

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Figure 1

Receiver operating characteristic curve for elevated serum ACE in adult sarcoid uveitis. ACE, angiotensin-converting enzyme.

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Figure 2

Receiver operating characteristic curve for lymphopaenia in adult sarcoid uveitis.

The sensitivity for the combination of elevated sACE and lymphopaenia in the diagnosis of sarcoid uveitis was 18.9%, Sp was 99.0%, PPV was 73.9% and NPV was 89.5%. This is quite similar to lymphopaenia alone where the difference does not reach statistical significance (Hanley and McNeil test, p=0.431) but slightly different from elevated sACE alone where the difference reaches statistical significance (p=0.026). When solely examining the biopsy-proven sarcoid uveitis group, SE for the combination of elevated sACE and lymphopaenia was 23.7%, Sp was 98.6%, PPV was 60.9% and NPV was 93.6%.

The sensitivity for the combination of lymphopaenia and/or elevated sACE in the diagnosis of sarcoid uveitis was 49.3%, Sp was 86.3%, PPV was 41.8% and NPV was 89.5%.

When using the optimal cut-off determined through the ROC curves (ie, 1.74 g/L for lymphopaenia and 61.2 IU/L for sACE), SE for elevated sACE in the diagnosis of sarcoid uveitis was 70.5%, Sp was 72.4%, PPV was 44.0%, and NPV was 88.8%. The SE for lymphopaenia in the diagnosis of sarcoid uveitis was 62.2%, Sp was 63.4%, PPV was 31.3% and NPV was 86.7%. The sensitivity for the combination of elevated sACE and lymphopaenia in the diagnosis of sarcoid uveitis was 49.5%, Sp was 89.9%, PPV was 64.1% and NPV was 82.9%.

COMPARISON OF RELIABILITY INDICES BY DEMOGRAPHICS AND ANATOMIC TYPES

When focusing on patients’ ages, for patients aged ≤50 years, SE for the combination of elevated sACE and lymphopaenia was 31.3%, Sp was 99.7%, PPV was 90.9% and NPV was 94.3%. For patients aged >50 years, SE for the combination of elevated sACE and lymphopaenia was 12.1%, Sp was 73.7%, PPV was 58.3% and NPV was 21.5% (Hanley and McNeil test, p=0.230). Thus, the difference does not reach statistical significance.

When studying anatomoclinical entities, for granulomatous uveitis SE for the combination was 26.2%, Sp was 97.3%, PPV was 73.3% and NPV was 82.5%; while for bilateral uveitis SE for the combination was 20.3%, Sp was 98.3%, PPV was 77.8% and NPV was 86.2%.

DISCUSSION

This study covers a large number of subjects presenting with unexplained uveitis in whom serum ACE and lymphocyte counts were both routinely measured.

The principle of such a biomarker combination was first proposed for sACE and lysozyme levels by Baarsma et al in 1987, while studying 221 patients with ocular sarcoidosis.²⁸

Based on a study of 191 patients with a first uveitis episode, Groen-Hakan et al found that lymphopaenia was strongly associated with a diagnosis of sarcoidosis, with an optimal cut-off of 1.47×10⁹/L, a strong sensitivity of 75% and a specificity of 77%.¹⁵ Their study excluded patients undergoing immunosuppressive therapies. In our study, we found a low sensitivity of 15.3% which may be explained by the fact that we used a lower cut-off of 1.0×10⁹/L. Our cut-off is the same as that used by Jones et al who found a sensitivity of 31.6% in their study based on 112 patients with sarcoid uveitis.¹⁶

Regarding the serum ACE elevation, Niederer et al found a high sensitivity of 78.1% whereas our study found 45.8%.¹³ These authors found a specificity of 90.0%, quite similar to our study (88.8%). They also found very high NPV of 97.0% for normal ACE in subjects presenting with undifferentiated uveitis, whereas our NPV was 89.2%. However, the PPV found by Niederer et al was only 43.6% and our study’s PPV for ACE was not much better (44.9%). This difference may relate to different diagnostic criteria and different levels of aggressiveness in pursuing the sarcoidosis diagnosis. Moreover, in Sims and Niederer study, the HRCT was not routinely performed, and more borderline cases may have been picked-up in our study, thus explaining the lower sensitivity of sACE.

Predictive values indicate the probability of disease in a patient with a positive test result (PPV) or the probability of no disease in a patient if the test is negative (NPV). Since conventional diagnostic tests have low PPV values in diagnosing sarcoid uveitis, a search for a more sensitive and specific diagnostic test is warranted.

Our study showed that combining elevated serum ACE and lymphopaenia makes it possible to enhance specificity to 99.0%, PPV to 73.9% and NPV to 89.5% for diagnosing sarcoid uveitis. This means that if both parameters are present, sarcoid uveitis is present in nearly three quarters of all cases. On the other hand, if both parameters are absent, one might miss diagnosing sarcoid uveitis in about 10% of all cases. However, based on our data, 50.7% of sarcoidosis patients will have negative sACE and no lymphopaenia; hence, the clinical picture remains the cornerstone of sarcoidosis diagnosis. Our study also shows that if the combination of both parameters occurs in patients aged <50 years old, sensitivity increases to 31.3%, specificity reaches 99.7%, PPV 90.9% and NPV 94.3%. Moreover, when the uveitis is granulomatous, sensitivity increases to 26.2%, specificity reaches 97.3%, PPV 73.3% and NPV 82.5%.

The strengths of our study were, first, that HRCT was frequently performed since it was performed in the 144 patients diagnosed with sarcoid uveitis. In previous studies, the HRCT was performed at physician’s discretion.¹³ Several studies have demonstrated the improved sensitivity of this examination over chest X-rays.²⁹ Niederer and Sims established the usefulness of screening investigations for systemic sarcoidosis in 709 subjects presenting with undifferentiated uveitis and found a sensitivity and specificity highest for chest CT than chest X-ray (98% and 100% vs 57.6% and 100%, respectively).¹⁷ Moreover, Kaiser et al showed that chest CT performed in older female patients with chronic uveitis showed mediastinal lymphadenopathy and other lesions suggestive of sarcoidosis in 17 patients (57%).³⁰ Second, we did not apply the International Workshop on Ocular Sarcoidosis (IWOS) 2009 criteria, since they have been called into question. Indeed, a study by Acharya et al showed that IWOS criteria had low sensitivities in diagnosing ocular sarcoidosis, with many suspected sarcoidosis patients who did not fit into the classification system.³¹ They found that although the IWOS clinical and investigational signs were significantly more often present in ocular sarcoidosis cases compared with control groups, laboratory investigations had low sensitivities, ranging from 5% for an abnormal liver enzyme test to 70% for anergy. Moreover, 37% of patients with suspected ocular sarcoidosis did not meet IWOS criteria. In the recently revised IWOS criteria, Mochizuki et al have changed the diagnostic protocol³²: the following four items were adopted by two-thirds of the panel members and instated as IWOS consensus agreement: (a) lymphopaenia, (b) elevated CD4/CD8 ratio (>3.5) in BAL fluid, (c) parenchymal lung changes consistent with sarcoidosis as determined by pulmonologists or radiologists and (d) abnormal label uptake on 67 Ga scintigraphy or 18 F-fluorodeoxyglucose PET imaging. However, this study was composed of a question-based survey and consensus meeting, based on personal experience of panel members.

Finally, our study has some limitations. First, this study is retrospective and monocentric, and includes an indication bias because diagnostic investigations were not performed in all cases and were performed at the discretion of the physicians, leading to a difference in diagnostic criteria making our study more difficult to compare with other studies. Moreover, we had to deal with missing data. Second, since our centre is a tertiary referral centre for uveitis management, some patients already had a long history of uveitis before being referred to our department and thus might differ from daily-care patients. As a result, this can make an important difference as sACE is often high and lymphocytes low during the acute phase, while they are not later in the disease course. However, this point made it possible for the study to include patients with lengthy follow-up after initial diagnosis. Third, patients in this study were referred to internal medicine practitioners by ophthalmologists with no precise pre-established criteria, beyond having an ophthalmological presentation suggestive of systemic disease. This could bias patient selection towards over-representation of systemic diseases. Finally, another limitation of our study relies in the use of different diagnostic criteria making it difficult to compare with previous studies.

CONCLUSION

Our study indicates that the combination of lymphopaenia and elevated serum ACE can serve as a useful biomarker for diagnosing sarcoidosis in uveitis patients, demonstrating a better overall diagnostic performance than solely that of serum ACE levels or lymphopaenia alone. This holds particularly true for young patients and those suffering from granulomatous uveitis.