Measurement of neopterin concentrations in body fluids like blood serum, cerebrospinal fluid or urine provides information about activation of cellular immune activation in humans under the control of T helper cells type 1. High neopterin production is associated with increased production of reactive oxygen species, neopterin concentrations also allow to estimate the extent of oxidative stress elicited by the immune system. 
Increased neopterin production is found in, but not limited to, the following diseases:
*Viral infections including human immunodeficiency virus (HIV), hepatitis B and hepatitis C, SARS-CoV-1, SARS-CoV-2.
*Bacterial infections by intracellular living bacteria such as Borrelia (Lyme disease), Mycobacterium tuberculosis, and Helicobacter pylori.
*parasites such as Plasmodium (malaria)
*Autoimmune diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE)
*Malignant tumor diseases
*Allograft rejection episodes.
*A leukodystrophy called Aicardi-Goutieres syndrome
*Depression and somatization.
Neopterin concentrations usually correlate with the extent and activity of the disease, and are also useful to monitor during therapy in these patients. Elevated neopterin concentrations are among the best predictors of adverse outcome in patients with HIV infection, in cardiovascular disease and in various types of cancer.
In the laboratory it is measured by radioimmunoassay (RIA), ELISA, or high-performance liquid chromatography (HPLC). It has a native fluorescence of wavelength excitation at 353 nm and emission at 438 nm, rendering it readily detected.