What it is
The absolute lymphocyte count (ALC) is the actual number of lymphocytes per microlitre of blood. A complete blood count reports lymphocytes as a percentage of the differential, but percentages move with the total white count, so the absolute number is what drives interpretation of lymphopenia and lymphocytosis. ALC is also the first-pass screen that flags an infant who may need an immunodeficiency work-up.
Formula
$$ ALC,(/\mu L) = WBC,(\times10^9/L) \times \frac{lymph,%}{100} \times 1000 $$
The calculator takes the WBC in ×10⁹/L (so a WBC of 8 = 8000 /µL) and the lymphocyte percentage, and returns the count in cells per microlitre.
When to use it
- Quantifying lymphopenia (immunosuppression, steroids, viral suppression, congenital immunodeficiency) or lymphocytosis (pertussis, viral infection, infants at baseline).
- Screening an infant in whom a persistently low ALC should trigger evaluation for SCID and other combined immunodeficiencies.
- Tracking lymphocyte recovery after chemotherapy or marrow transplant.
- As a low-cost surrogate for CD4 trends and a trigger to send formal lymphocyte subset flow cytometry when the count is persistently abnormal.
Worked example
A child has a WBC of 8 ×10⁹/L with 30% lymphocytes.
$$ ALC = 8 \times \frac{30}{100} \times 1000 = 2400\ /\mu L $$
An ALC of 2400/µL sits in the broad normal band for an older child. In a young infant, however, the same number is on the low side — infant lymphocyte counts normally run roughly 4000–10,000/µL under one year of age, so a persistently low ALC (e.g. <2500/µL) in an infant should prompt evaluation for SCID/immunodeficiency.
Pitfalls
- Age, age, age. Reference ranges are strongly age-dependent. Applying adult or older-child cut-offs to a neonate or young infant is the single most common error — infants are physiologically lymphocyte-rich.
- The SCID caveat. A low ALC in an infant is a red flag, not reassurance. Many newborn screening programmes use TREC assays, but a low ALC on a CBC remains a key bedside trigger to escalate.
- Percentage vs absolute. A “normal looking” lymphocyte percentage can hide a low absolute count when the total WBC is depressed (or vice versa). Always interpret the absolute number.
- Single value vs trend. One isolated value rarely settles a diagnosis; persistence over serial counts is what matters.
- Garbage in. A spuriously high WBC (clumping, nucleated RBCs miscounted) propagates straight into the ALC. Sanity-check the analyser flags.
Run it: [Absolute Lymphocyte Count (ALC)](/calculator/absolute-lymphocyte-count)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.