Age‐Related Decline in Lymphocyte Counts: Establishing Age‐Specific Reference Intervals for Clinical Practice

Applying a fixed lymphocyte count reference interval to all adults, potentially misclassifying healthy elderly individuals as lymphopenic and contributing to unnecessary investigations and anxiety. The absolute lymphocyte count (ALC) is a critical marker of immune competency and is routinely used to assess both acute and chronic immunological disorders [1]. Despite well-recognized biological aging processes, most UK laboratories employ fixed ALC reference intervals (generally 1.0–4.0 × 109/L) for all persons over 12 years. These fixed thresholds neglect the well-established phenomenon of immunosenescence, wherein ALC progressively declines with age due to thymic involution and changes in lymphoid cell output and subsets [2, 3]. For example, in East Sussex, UK, a fixed lower limit of 1.5 × 109/L is applied across all adults. However, this practice risks misclassifying a substantial proportion of otherwise healthy elderly individuals as lymphopenic, which can trigger unnecessary investigations, specialist referrals, patient anxiety, and increased strain on healthcare resources. To address this knowledge gap, we retrospectively analyzed 45 780 full blood counts (FBCs) from individuals aged 0 to 102 years in East Sussex between October and December 2024, focusing on 44 339 adults aged 18 years and above. Only results within the standard laboratory reference range were included, using this as a surrogate for general health. Our cohorts were stratified into eight age groups (≤ 17, 18–29, 30–39, 40–49, 50–59, 60–69, 70–79, and ≥ 80 years, Figure S1), and we acknowledge the distinction between pediatric and adult intervals, referencing established UK pediatric reference intervals [4]. Our analysis adheres to Clinical and Laboratory Standards Institute (CLSI) and International Federation of Clinical Chemistry (IFCC) guidelines for deriving age-specific reference intervals using non-parametric methods modeling for right-skewed ALC distributions (Figure S2) [5]. The central 95% reference interval was defined by the 2.5th and 97.5th percentiles of the distribution in each age group. Linear and polynomial regression models were used as illustrative to describe the overall trend. Sex differences were assessed using regression models. False-positive lymphopenia rates were compared between fixed and age-specific cutoffs. All analyses were IRB-approved (Eastbourne District General Hospital, Approval Number: 1784) and performed using IBM SPSS Statistics (v28, MacOS). We demonstrate a robust age-associated decline in ALC: the mean value across the population was 1.95 ± 0.72 × 109/L (median 1.88 × 109/L, IQR 1.47–2.36 × 109/L, Figure 1). ALC displayed a significant negative correlation with age (Pearson r = −0.242, p < 0.001; Spearman rho = −0.24, p < 0.001, Table S1), with the largest declines observed when comparing the youngest and oldest populations (mean difference: −0.80 × 109/L, p < 0.001, Figure S3a,b). Linear regression shows each year of age predicts a decline of 0.009 × 109/L, corresponding to about 0.09 × 109/L per decade, or a roughly 4%–5% decrease per decade. The regression-based estimation formula: Predicted ALC = 2.470–(0.009 × Age); allows clinicians to evaluate expected ALC by age. Females had slightly higher ALC than males (β≈0.05, p < 0.001). However, the effect size was minimal. The age-by-sex interaction term was not significant, confirming that both sexes exhibit similar rates of ALC decline with age (Figure S4). Table 1 provides the age-stratified reference intervals derived from our cohort. For example, the lower 2.5th percentile for the 18–29 age group is 1.01 × 109/L, compared to only 0.59 × 109/L in those 80 years and older. Thus, ALC results flagged as “low” in younger adults may be entirely normal for those in later life. Crucially, we found that applying a fixed lower ALC cutoff (e.g., 1.5 × 109/L) markedly overestimates lymphopenia in older age groups: 9.2% of individuals aged 18–29 years versus 32.2% of those aged ≥ 80 years. In contrast, implementing age-specific 2.5th percentile cutoffs reduced the rate of false-positive lymphopenia, for example from 32.2% to 27.2% in individuals aged ≥ 80 years; a 5% absolute risk reduction. In some middle-aged groups, relative risk reduction of false positives reached 60%. These observations emphasize that lymphopenia defined by fixed adult reference intervals is not an accurate indicator of pathology in older adults but rather often reflects benign, physiological immunosenescence. The data support replacing uniform adult reference intervals with robust, age-specific ALC reference intervals that more accurately reflect normal hematologic aging. This approach will enhance diagnostic precision, reduce unnecessary referrals and investigations, and alleviate patient anxiety brought on by misinterpretation of lab results. While the 2.5th percentile has been used here as a statistical cutoff to define potentially low ALC within each age group, it remains a proposed threshold rather than a validated diagnostic criterion. This demarcation is based on the distributional properties of ALC in our cohort and aligns with established practices for defining reference intervals (Table S2). However, further prospective clinical studies are necessary to confirm the appropriateness of this cutoff for diagnosing pathological lymphopenia. Until such validation is available, clinicians are advised to interpret ALC below this threshold with clinical correlation and adjunctive investigations as appropriate. Our findings are reinforced by international data; other large epidemiologic studies also report that lower ALC is typical in older adults, supporting efforts to revise clinical practice guidelines and laboratory information systems accordingly [6-8]. Fixed-threshold approaches would label about one-third of the very elderly individuals as lymphopenic, highlighting the risk of overdiagnosis and its consequences in geriatric populations. It is important to note, however, that even “normal-for-age” low ALC among the elderly may have clinical implications, as scoping reviews link lower ALC in older adults with frailty and increased risk of adverse outcomes, including infection [8]. This highlights the dual role of ALC: first, as age-referenced laboratory markers to avoid overdiagnosis of lymphopenia, and second, as potential prognostic indicators of health risk within the older population. Both aspects should guide clinical interpretation. Additional studies are required to investigate whether and how these thresholds should inform clinical assessment and preventative strategies in geriatrics, such as infection risk, vaccine response, and their integration into frailty or nutritional scores (e.g., HALP score) [8]. This study's strengths include its large, unselected UK sample, rigorous application of CLSI/IFCC methodology, comprehensive age stratification, and robust non-parametric statistics, making the results broadly generalizable. Key limitations are its single-center, retrospective design, exclusion of detailed lymphocyte subset analysis (e.g., CD4+/CD8+, B cells, NK cells), and reliance on single point-in-time FBCs as a proxy for health. While this provides a clinically realistic population, prospective linkage with clinical outcomes would further strengthen the evidence base. Furthermore, inter-individual differences in ALC due to biological (sex, ethnicity, and health status) and environmental (smoking, stress, and infection) factors, although typically modest, should be considered for borderline results [9, 10]. To maximize the clinical impact of these findings, we are currently partnering with other NHS Trusts and academic institutions to validate the proposed intervals across more diverse populations and assess their effect on clinical decision-making, healthcare resource use, and patient outcomes. The integration of age-specific ALC cutoffs into laboratory reporting systems and clinical practice guidelines will advance evidence-based patient care. Automated age-adjusted reference intervals should be incorporated within laboratory information systems to deliver more accurate and personalized results to clinicians. Increasingly, patients are gaining direct access to their own laboratory data through secure online portals, empowering them to be more engaged in their health management. Providing patients with age-adjusted reference ranges helps ensure they receive clear, relevant information tailored to their individual profiles, thereby supporting shared decision-making between patients and healthcare providers. In conclusion, these data demonstrate that a fixed adult lower limit for ALC is outdated and results in frequent misclassification of lymphopenia in the elderly. We demonstrated that employing age-tailored reference limits for ALC could substantially cut down false positive abnormal results in clinical practice, especially among older adults. Clinical laboratories, national bodies, and guideline committees should urgently update recommendations to incorporate age-specific ALC reference values. This change will enhance diagnostic accuracy, reduce unnecessary patient anxiety and healthcare utilization, and align laboratory practices with current evidence on the immunology of aging. This would parallel the precision seen in pediatric ranges and ultimately lead to more accurate interpretation of laboratory tests throughout a patient's life. We thank the Department of Hematology and the laboratory staff at Eastbourne District General Hospital for their support. The authors declare no conflicts of interest. Data are available from the corresponding author on reasonable request. Figure S1: Age Group Frequency Bar Chart. Figure S2: Q-Q Plot of Lymphocyte Counts (×109/L) illustrates the data distribution. Figure S3a: Q-Q Plot of Lymphocyte Counts (×109/L) for Age Group ≤ 17 Years vs. ≥ 80 Years. Figure S3b: Histogram of Lymphocyte Counts (×109/L) for Age Group ≤ 17 Years vs ≥ 80 Years. Figure S4: Boxplots lymphocyte counts (×109/L) grouped by gender. The central line in each box represents the median lymphocyte count, while the lower and upper edges of the box denote the 25th (Q1) and 75th (Q3) percentiles, respectively. The whiskers extend to 1.5 times the interquartile range (IQR) from the quartiles. Points displayed as circles and stars outside the whiskers are outliers, which represent individual lymphocyte counts significantly higher or lower than the typical range for that gender group. Table S1: Descriptive Statistics for Lymphocyte Counts (×109/L) Across Age Groups. Table S2: Proposed age-specific reference intervals and Suspected Potentially Concerning Thresholds for Lymphocyte Counts (×109/L). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. 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