Lymphopenia and risk of infection and infection-related death in 98,344 individuals from a prospective Danish population-based study

Abstract

Background: Neutropenia increases the risk of infection, but it is unknown if this also applies to lymphopenia. We therefore tested the hypotheses that lymphopenia is associated with increased risk of infection and infection-related death in the general population.

Methods and findings: Of the invited 220,424 individuals, 99,191 attended examination. We analyzed 98,344 individuals from the Copenhagen General Population Study (Denmark), examined from November 25, 2003, to July 9, 2013, and with available blood lymphocyte count at date of examination. During a median of 6 years of follow-up, they developed 8,401 infections and experienced 1,045 infection-related deaths. Due to the completeness of the Danish civil and health registries, none of the 98,344 individuals were lost to follow-up, and those emigrating (n = 385) or dying (n = 5,636) had their follow-up truncated at the day of emigration or death. At date of examination, mean age was 58 years, and 44,181 (44.9%) were men. Individuals with lymphopenia (lymphocyte count < 1.1 × 109/l, n = 2,352) compared to those with lymphocytes in the reference range (1.1-3.7 × 109/l, n = 93,538) had multivariable-adjusted hazard ratios of 1.41 (95% CI 1.28-1.56) for any infection, 1.31 (1.14-1.52) for pneumonia, 1.44 (1.15-1.79) for skin infection, 1.26 (1.02-1.56) for urinary tract infection, 1.51 (1.21-1.89) for sepsis, 1.38 (1.01-1.88) for diarrheal disease, 2.15 (1.16-3.98) for endocarditis, and 2.26 (1.21-4.24) for other infections. The corresponding hazard ratio for infection-related death was 1.70 (95% CI 1.37-2.10). Analyses were adjusted for age, sex, smoking status, cumulative smoking, alcohol intake, body mass index, plasma C-reactive protein, blood neutrophil count, recent infection, Charlson comorbidity index, autoimmune diseases, medication use, and immunodeficiency/hematologic disease. The findings were robust in all stratified analyses and also when including only events later than 2 years after first examination. However, due to the observational design, the study cannot address questions of causality, and our analyses might theoretically have been affected by residual confounding and reverse causation. In principle, fluctuating lymphocyte counts over time might also have influenced analyses, but lymphocyte counts in 5,181 individuals measured 10 years after first examination showed a regression dilution ratio of 0.68.

Conclusions: Lymphopenia was associated with increased risk of hospitalization with infection and increased risk of infection-related death in the general population. Notably, causality cannot be deduced from our data.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. Flow chart of individuals from the Copenhagen General Population Study.

Fig 2. Regression toward the mean of lymphocyte counts, categorized as lymphopenia (9/l), reference lymphocyte count (1.1–3.7 × 109/l), and lymphocytosis (>3.7 × 109/l) at the date of examination in 2003 in the Copenhagen General Population Study.

When categorizing lymphocyte counts, lymphopenia was defined as a lymphocyte count below the 2.5th percentile, the reference category was defined as a lymphocyte count between the 2.5th and 97.5th percentile, and lymphocytosis was defined as a lymphocyte count above the 97.5th percentile. Median values and interquartile ranges of 5,181 individuals at the 2003 examination (left) and the same individuals—maintaining the 2003 categorization—at the 2013 examination (right). Regression dilution ratio is 2.04/2.98 = 0.68. The table below the graph gives the percentage of individuals in each lymphocyte category at the 2003 examination (rows) and at the 2013 examination (columns).

Fig 3. Risk of any infection as a function of lymphocyte count for individuals from the Copenhagen General Population Study.

Solid red lines are hazard ratios, and dashed black lines indicate 95% confidence intervals based on fitting of cubic splines to risk estimates obtained using Cox proportional hazards regression. Multivariable adjustment includes all covariates listed in Table 1 except age, but with age as the underlying timescale. The median lymphocyte value of 2.1 × 109/l was set as reference for the continuous model. For the multivariable adjusted model, the lymphocyte interval 30–40 × 109/l is added to illustrate that for very high lymphocyte counts the risk of infections increases, but with a broad confidence interval. When categorizing lymphocyte counts, lymphopenia was defined as a lymphocyte count below the 2.5th percentile, the reference category was defined as a lymphocyte count between the 2.5th and 97.5th percentile, and lymphocytosis was defined as a lymphocyte count above the 97.5th percentile.

Fig 4. Multivariable adjusted risks of specific infections as a function of lymphocyte count for individuals from the Copenhagen General Population Study.

Solid red lines are multivariable adjusted hazard ratios, and dashed black lines indicate 95% confidence intervals based on fitting of cubic splines to risk estimates obtained using Cox proportional hazards regression. Multivariable adjustment includes all covariates listed in Table 1 except age, but with age as the underlying timescale. The median lymphocyte value of 2.1 × 109/l was set as reference for the continuous model. The sum of the numbers of cases of specific infections exceeds the number of cases of “any infection” since individuals could have more than 1 specific infection. When categorizing lymphocyte counts, lymphopenia was defined as a lymphocyte count below the 2.5th percentile, the reference category was defined as a lymphocyte count between the 2.5th and 97.5th percentile, and lymphocytosis was defined as a lymphocyte count above the 97.5th percentile.

Fig 5. Multivariable adjusted risk of any infection for individuals from the Copenhagen General Population Study with lymphopenia (lymphocyte count 9/l) compared to individuals with lymphocytes in the reference range (1.1–3.7 × 109/l).

Multivariable adjustment includes all covariates listed in Table 1 except age, but with age as the underlying timescale. The sum number of individuals in the strata varies slightly because of varying numbers of individuals with missing data in the covariates. When categorizing lymphocyte counts, lymphopenia was defined as a lymphocyte count below the 2.5th percentile, while the reference category was defined as a lymphocyte count between the 2.5th and 97.5th percentile. NSNot significant after adjusting for 12 multiple comparisons using the Bonferroni method.aAccording to a likelihood ratio test.b≤168 versus >168 g/week for men and ≤84 versus >84 g/week for women.cWhen categorizing neutrophil counts, neutropenia was defined as a neutrophil count below 1.5 × 109/l, which is the generally accepted lower cutoff [,–43], the reference category was defined as a neutrophil count from 1.5 × 109/l to 5.9 × 109/l, and neutrocytosis was defined as a neutrophil count above 5.9 × 109/l.dAs defined by the Charlson comorbidity index.

Fig 6. Multivariable adjusted risk of any infection for individuals from the Copenhagen General Population Study with lymphopenia using age-adjusted percentiles of lymphocyte count.

Multivariable adjusted risk of any infection for individuals from the Copenhagen General Population Study with lymphopenia (lymphocyte count 9/l) compared to individuals with lymphocytes in the reference range (1.1–3.7 × 109/l). Multivariable adjustment includes all covariates listed in Table 1 except age, but with age as the underlying timescale. Individuals with lymphopenia had a lymphocyte count below the 2.5th percentile in each 1-year age band. aAccording to a likelihood ratio test.

Fig 7. Multivariable adjusted risk of any infection for individuals from the Copenhagen General Population Study with lymphopenia stratified by follow-up time.

Multivariable adjusted risk of any infection for individuals from the Copenhagen General Population Study with lymphopenia (lymphocyte count 9/l) compared to individuals with lymphocyte count in the reference range (1.1–3.7 × 109/l). Multivariable adjustment includes all covariates listed in Table 1 except age, but with age as the underlying timescale. Follow-up time was segmented into 3 time intervals, and each individual was included in more than 1 time interval if total follow-up was above 2 years. When categorizing lymphocyte counts, lymphopenia was defined as a lymphocyte count below the 2.5th percentile, while the reference category was defined as a lymphocyte count between the 2.5th and 97.5th percentile. aAccording to a likelihood ratio test.

Fig 8. Algorithm.

Absolute 2-year risks of hospitalization due to infection for different blood lymphocyte categories, stratified for age, sex, and smoking status. Nonsmokers were former smokers and never-smokers combined. Individuals diagnosed with immunodeficiency, hematologic disease, and/or autoimmune disease were not included when calculating absolute risks for the algorithm, since these individuals have a well-known high risk of infection. Numbers indicate absolute 2-year risk as rounded percentages.

Fig 9. Multivariable adjusted risk of infection-related death as a function of lymphocyte count for individuals from the Copenhagen General Population study.

The solid red line is the multivariable adjusted hazard ratio, and black dashed lines indicate the 95% confidence interval based on fitting of cubic splines to risk estimates obtained using Cox proportional hazards regression. Multivariable adjustment includes all covariates listed in Table 1 except age, but with age as the underlying timescale. The median lymphocyte value of 2.1 × 109/l was set as reference for the continuous model. When categorizing lymphocyte counts, lymphopenia was defined as a lymphocyte count below the 2.5th percentile, the reference category was defined as a lymphocyte count between the 2.5th and 97.5th percentile, and lymphocytosis was defined as a lymphocyte count above the 97.5th percentile.