Prevalence of the 8 HLH-2004 Criteria in HLH Subgroups
One hundred sixty-five HLH suspected patients were included in the primary cohort. After evaluating the HLH-2004 criteria, we had 79 non-HLH and 86 HLH patients (16 pHLH and 70 sHLH). The median age of the pHLH group was 0.5 (0–50.7) years, 8.7 (0–83) years in the sHLH group and 9.7 (0–84) years in the non-HLH group. The type of pHLH and the affected gene and genetic variants of the pHLH patients included in this study are reported in supplementary table 1 and included pathologic mutations in PRF1, UNC13D, STX11, and STXBP2. The most common primary diagnosis in the non-HLH group was either autoimmune disease (20.8%) or immunodeficiency (20.8%), whereas infection (27%) and malignancy (24.3%) were the most common causes of sHLH (Table 1). In 62% of the HLH cases (ranging from 58.8 to 100% among subgroups), hemophagocytosis was observed, either in bone marrow aspirates and/or other affected tissue. Patients with sHLH induced by autoimmune disease fulfilled the cytopenia criterion less often and patients with sHLH induced by infections more often had normal NK lysis assays, suggesting that there was significant variation in positive criteria between HLH subgroups. Moreover, significant variation also existed within the HLH subgroups, denoted by the large standard deviations that were found. This was underlined by the fact that the MHscore, a diagnostic tool that differentiates pHLH from MAS, could not differentiate between pHLH and sHLH in this cohort. [23] The score showed an AUC of 0.709, pHLH patients had a median score of 78.5 (60–99.25), and sHLH patients had a median score of 60 (34–72). To identify common denominators of HLH between these groups, we first tried a steered approach, followed by several clustering methods.
Table 1 Summary statistics on included patients. Non-HLH patients were defined as patients who did not meet the HLH-2004 criteria. The type of underlying disease was defined as the primary diagnosis that the patient was suffering from (e.g., a patient suffering from SLE and infection was scored as autoimmune; AI) The largest variation in symptom positivity was seen in the presence of cytopenias and an aberrant NK lysis assay. Moreover, splenomegaly, cytopenia, and elevated ferritin were frequently encountered in HLH patients The steered approach consisted of an evaluation of parameters that are readily available in the clinical setting: elevated ferritin, cytopenias in ≥2 cell lines, and splenomegaly. We hypothesized that these criteria may guide decision-making towards additional HLH diagnostics. First, we calculated the diagnostic properties of ferritin as sole marker for HLH. We set the sensitivity to 90% and found that a ferritin value 787 μg/L enabled us to identify patients with HLH with 48% specificity and an AUC of 0.693. To improve this, we then added the other criteria and observed that by adding splenomegaly or severe cytopenia in >2 lineages (according to the HLH-2004 criteria) to the model, the cutoff for ferritin could be increased to 1000 μg/L. The presence of either splenomegaly, severe cytopenia in >2 lineages (according to the HLH-2004 criteria), or increased ferritin (>1000 μg/L) yielded 100% sensitivity and 65% specificity with a negative predictive value of 100% for HLH in the discovery cohort (Supplementary, table 2).
Hierarchical Clustering
Different clustering analysis methods were used to determine the parameters that could distinguish between HLH and non-HLH patients and the different subtypes of HLH. First, hierarchical clustering was applied to the 8 HLH-2004 criteria, CNS symptoms, NK lymphocyte numbers, and bilirubin levels. We found that even though no specific clusters were formed, hierarchical clustering was moderately capable at separating the non-HLH patients from the HLH patients (Fig. 1A). However, hierarchical clustering did not separate the pHLH from sHLH patients (Fig. 1B), nor other subgroups within the HLH spectrum. This suggests that there is no specific set of parameters that can distinguish the separate forms of HLH within the HLH-2004 criteria.
Supervised Clustering with Dimension Reduction
Secondly, PLS-DA was used to maximize the chance of finding discriminating clusters of criteria that could define the sHLH subgroups and distinguish between sHLH and pHLH, but none could be found (Supplementary, fig. 1). The criteria that could identify HLH patients in general overlapped excessively for the pHLH and sHLH patients and also for other HLH subgroups. Hence, all subgroups were pooled for further analysis.
The results of the pooled PLS-DA are shown in Fig. 2A and 2B. The combination of the presence of splenomegaly, together with cytopenias, proven tissue hemophagocytosis, fever, increased sCD25, and elevated triglycerides could distinguish HLH patients from non-HLH patients effectively with an area under the curve (AUC) of 0.93. Moreover, splenomegaly, biopsy-proven hemophagocytosis, and cytopenias are the most distinguishing parameters in this analysis.
Unsupervised Clustering with Dimension Reduction
Finally, a PCA analysis with oblique rotation was performed to see if an unsupervised approach would yield similar results as the supervised approach. The Kaiser-Meyer-Olkin (KMO) measure showed that there were too few samples to explain all variables, which led to the exclusion of NK lysis 2:1, NK fold change and age, since these had the poorest common variance. This resulted in an overall KMO = 0.64 with no single value below 0.5. Bartlett’s test of sphericity resulted in chi-squared (45) = 239, p < 0.0001. The scree plot showed an “elbow” at four factors, indicating that four factors were sufficient to explain most variance within the dataset. This was confirmed by the dimension reduction data which showed that four factors had a cumulative variance of 67%, of which the first three explained 81% (Supplementary figure 2 & Supplementary table 3).
Cluster analysis showed that either PC1 or PC2 combined with PC3 could distinguish between the non-HLH and HLH patients (Fig. 2C). The variance in PC1 was mostly caused by fever (0.74), triglycerides (0.65) and splenomegaly (0.55), and biopsy-proven phagocytosis (0.55). For PC2, this was ferritin (0.84), NK lysis (0.96), and sCD25 (0.35) and for PC3, leukocytes (0.98), neutrophils (0.95), and platelets (0.55).
Simulating Minimal Parameter Sets with multiROC
Since tissue hemophagocytosis, splenomegaly and cytopenias were defining parameters for HLH in both the PLS-DA and the PCA; these criteria were used as initial parameter set. We then simulated the minimal parameter set needed for HLH diagnosis, by iteratively adding the criterion that caused the largest increase in AUC (Supplementary, figure 3). This ultimately led to the discovery of a combination of biopsy-proven hemophagocytosis, splenomegaly, cytopenias in ≥2 lineages, ferritin ≥1000, and ⇑triglycerides/⇓fibrinogen with an AUC of 0.95. Further addition of the other criteria (fever, sCD25, and aberrant NK/lymphocyte function assay) did not greatly improve the algorithm (Fig. 2D, AUC 0.96–097), suggesting that these parameters are not essential for the diagnosis of HLH.
Furthermore, since splenomegaly and biopsy-proven hemophagocytosis clustered together in the PCA and were also among the top discriminative parameters in the PLS-DA, these criteria were analyzed as major criteria. The remaining three criteria were analyzed as minor criteria and compared to the golden standard, the HLH-2004 criteria, as presented in Table 2. HLH was most likely when a patient either had 2 major positive criteria (48% sensitivity, 100% specificity), 1 major and 2 minor positive criteria (79% sensitivity, 95% specificity), or 3 minor positive criteria (49% sensitivity, 97% specificity), with a combined sensitivity of 94% and specificity of 95% (Table 2 and Fig. 3).
Table 2 Analysis of the sensitivity and specificity of the minimal parameter set that can predict HLH with splenomegaly and tissue phagocytosis as major criteria and ferritin, cytopenia, and triglycerides/fibrinogen as minor criteria. These were replicated in another retrospective cohort which produced similar results Analysis of the Minimal Parameter Set in the Replication Cohort
The replication cohort consisted of 109 sHLH patients with a median age of 58 (19–77) and 38 non-HLH patients with a median age of 54 (19–81). The most common primary diagnosis in the non-HLH group was autoimmune (47%), whereas malignancy (39.4%) and infection (36.7%) were the most common causes of sHLH in this cohort (Supplementary, table 4).
The minimal parameter set, which distinguished patients with HLH from non-HLH patients when two major criteria were positive (44% sensitivity, 100% specificity), one major and two minor criteria were positive (87% sensitivity, 97% specificity), or three minor criteria were positive (76% sensitivity, 97 specificity), could distinguish sHLH patients from non-HLH patients with 98% sensitivity and 95% specificity which confirmed the sensitivity and specificity of the minimal parameter set.
Furthermore, the presence of either splenomegaly, cytopenias in ≥2 cell lines, or ferritin ≥1000 μg/L yielded 100% sensitivity and 16% specificity.
The Role of the NK/Lymphocyte Function and sCD25 Assays
Even though the NK/lymphocyte function and sCD25 assays are not included in the minimal parameter set, they are part of the HLH-2004 diagnostic criteria. We measured their performance in this cohort as decisive fifth criterion in borderline positive cases. There were 26 cases in which HLH was diagnosed based on the minimum of 5 positive criteria. Since sensitivity of the NK/lymphocyte function assay was low (Table 3), it could only be used as fifth positive criterion in 6/26 cases. Hence, we calculated a cutoff for the dilution series (n = 103, 62 non-HLH patients and 41 HLH patients) with maximum sensitivity at a specificity of at least 90%, to improve the diagnostic properties of the NK lysis assay, without impairing its robustness. This wielded a cutoff fold change of 1.17 with an AUC of 0.602, which significantly improved the diagnostic properties of the NK/lymphocyte function assay (Table 3).
Table 3 Analysis of the performance of the current NK lymphocyte function tests vs addition of the dilution series as sole predictor for HLH (*p < 0.01) and sensitivity and specificity cutoffs for sCD25 fold change as HLH predictor sCD25 has previously been suggested as a sensitive HLH marker. [24] In our cohort, it was needed to get to 5 positive criteria in 20/26 cases. To confirm previous findings, the performance of sCD25 as sole indicator of HLH was measured in our cohort (n = 120, 59 non-HLH patients, 61 HLH patients). Cutoffs were calculated with a minimum sensitivity of 90% and with a minimum specificity of 90%, which were 2.63 and 11.8 respectively with an AUC of 0.806 (Table 2). These results implicate that although NK lysis and sCD25 are not needed for initial treatment initiation, they can be used to acquire the five positive criteria needed for unambiguous diagnosis or to further support the diagnosis of HLH.