Reproducibility of CSF quantitative culture methods for estimating rate of clearance in cryptococcal meningitis

Quantitative cerebrospinal fluid (CSF) cultures provide a measure of disease severity in cryptococcal meningitis. The fungal clearance rate by quantitative cultures has become a primary endpoint for phase II clinical trials. This study determined the inter- assay accuracy of three different quantitative culture methodologies. Among 91 participants with meningitis symptoms in Kampala, Uganda, during August-November 2013, 305 CSF samples were prospectively collected from patients at multiple time points during treatment. Samples were simultaneously cultured by three methods: (1) St. George's 100 mcl input volume of CSF with five 1: 10 serial dilutions, (2) AIDS Clinical Trials Group (ACTG) method using 1000, 100, 10 mcl input volumes, and two 1: 100 dilutions with 100 and 10 mcl input volume per dilution on seven agar plates; and (3) 10 mcl calibrated loop of undiluted and 1: 100 diluted CSF (loop). Quantitative culture values did not statistically differ between St. George-ACTG methods (P = .09) but did for St. George-10 mcl loop (P < .001). Repeated measures pairwise correlation between any of the methodswas high (r >= 0.88). For detecting sterility, the ACTG-method had the highest negative predictive value of 97% (91% St. George, 60% loop), but the ACTG-method had occasional (similar to 10%) difficulties in quantification due to colony clumping. For CSF clearance rate, St. George-ACTG methods did not differ overall (mean - 0.05 +/- 0.07 log(10)CFU/ml/day; P = .14) on a group level; however, individual-level clearance varied. The St. George and ACTG quantitative CSF culture methods produced comparable but not identical results. Quantitative cultures can inform treatment management strategies.