Figure 4

Preservation of protease activity in a preanalytical time period of 24 h. Aliquots of serum specimens from 6 tumor patients were frozen at −80°C directly after centrifugation (0 h) or after prolonged preanalytical time span of 3 h, 6 h, and 24 h. After thawing, specimens were spiked with CP-RP and incubated for 22 h prior to peptide extraction with TCA and LC-MS. CP-AP peak areas were extracted from the data. The CP-AP concentrations of the freshly obtained serum aliquots (0 h) were set to 100%. In the box plot the central box represents the values from the lower to upper quartile (25 to 75 percentile). The middle line represents the median. The horizontal line extends from the minimum to the maximum value. P-values of the Mann–Whitney test are indicated.