Category: Critical Care
Keywords: Neutrophils, Lymphocytes, NLR, ANC, ALC, Sepsis, COVID (PubMed Search)
As has been previously noted, the white blood cell count is "the last refuge of the intellectually destitute." However, within a CBC (especially if a differential is obtained), there is information that can sometimes be of value. One measure, which was noted before COVID but has come under increasing attention in the current pandemic, is the Neutrophil-To-Lypmhocyte Ratio (NLR). Because physiologic stress typically causes the Absolute Neutrophil Count (ANC) to increase and the Absolute Lymphocyte Count (ALC) to decrease, the ratio of the two values (NLR = ANC/ALC) should increase when the body is under stress. Similar to the WBC however, it should be noted that ANY source of physiologic stress can cause abnormalities of the NLR, and thus this is not limited strictly to infectious etiologies.
With that caveat in mind, the NLR can sometimes be a clue to the degree of physiologic stress the patient is under. As lymphopenia is a frequent finding in COVID, the NLR has come under particular interest in the setting of COVID and appears to have prognostic value in COVID+ patients.
It should be kept in mind that inflammatory stressors (e.g. sepsis) are likely to disproportionately raise the NLR relative to noninflammatory stressors (e.g. pulmonary embolism), so a septic patient with an NLR of 10 might not be all that ill, whereas a PE patient with an NLR of 10 may be sicker. As with any single lab, and particularly one so nonspecific, there are no hard and fast cutoffs, and the NLR has to be interpreted in the context of other clinical data (it is very much possible to have a high NLR and not be that sick, or to have a low NLR and be sick... this is only one datapoint and does have pitfalls associated with it). As a rough guide however, a Pulmcrit post by Josh Farkas from 2019 suggested the following interpretation of the NLR:
6-9: Mild stress (e.g. uncomplicated appendicitis)
9-18: Moderate stress, may be associated with critical illness
>18: Severe stress, commonly associated with critical illness
The post (see references below) provides an excellent overview of NLR, further information on the uses and pitfalls of NLR, and several additional sources on the subject. It's a very worthwhile read.
Bottom Line: The Neutrophil-To-Lymphocyte Ratio (NLR = ANC/ALC) is one indicator of the degree of physiologic stress, and may be used in conjuction with other clues to determine how sick your patient is.