Functional titer is defined as the number of functional vector particles required to infect a cell, present in a volume. There are several methods for measuring titer. Some are more reliable than others. Other methods include: measuring the p24 concentration/ml, measuring RNA equivalents, Transducing units/ml, or measuring mRNA equivalents. The first two are unreliable because they tend to overestimate the vector titer. The amount of p24 measured comes from functional particles, free p24, and nonfunctional vector particles. The RNA assays measure defective particles as well.

Reliable methods are determined by transduction of cells following limiting dilution of vector and subsequent evaluation of reporter protein activity (eGFP) or by the number of colonies following antibiotic selection.

The most straightforward method is to quantify functional vector titer by employing fluorescence and FACs. This is the method employed by the ViraCore. The method does have some limitations including being restricted to fluorescent reporter proteins and it cannot distinguish between single or multiple integrations. This method is best because it only accounts for functional particles. Titers are calculated from infection of live cells in a 96 well plate. The formula used is: (#cells X %infected cells) X 1,000 μl/virus (ml). It is reported in TU/ml which translates to Transducable Units per milliliter.