Effect of pH changes in cerebrospinal fluid specimens on bacterial survival and antigen test results.

AIMS

To determine the effect of pH changes occurring in cerebrospinal fluid (CSF) after sampling on the viability of meningitis causing bacteria, and on the performance of agglutination assays used for the rapid detection of bacterial antigens.

METHODS

The pH of CSF collected via lumbar puncture was measured by various methods, and the effects of the following different incubation conditions on subsequent changes were determined: air at 4 degrees C; air at room temperature (22 degrees C); air at 37 degrees C; and air with 5% CO2 at 37 degrees C. The growth/survival in pooled CSF of 15 bacterial isolates collected from 74 patients with meningitis was assessed in these incubation conditions over 24 hours. The effects of pH changes in the CSF on the sensitivity of two latex agglutination and one co-agglutination kits for detecting Haemophilus influenzae, Neisseria meningitidis groups B and C, and Streptococcus pneumoniae were determined.

RESULTS

The measured pH of CSF was highly affected by the method used and particularly the time delay between patient sampling and assay. Measured pH values at the time of sampling (mean 7.5) increased rapidly within 60 seconds by about one unit. CSF pH continued to increase during incubation in all tested conditions (up to approximately pH 10), with the exception of in air with 5% CO2 at 37 degrees C where pH changes were reversible and near physiological values were attained. Bacterial survival for all species tested was poorest in CSF incubated in air at 37 degrees C and best following exposure to air with 5% CO2 at 37 degrees C. Agglutination in rapid antigen detection kits with CSF incubated in air as opposed to air with 5% CO2 generally took longer to occur and in some instances was less prominent. In one case a false negative result was obtained with CSF seeded with N meningitidis group B incubated in the former but not the latter conditions.

CONCLUSIONS

CSF pH increases after patient sampling are minimised and/or mostly reversed by incubation in an atmosphere containing 5% CO2. CSF samples should ideally be placed in such an atmosphere as soon as possible after collection, and left there until laboratory processing occurs, to reduce the detrimental effects of pH stress on bacterial survival. pH increases may also reduce the likelihood of obtaining a positive result in rapid antigen detection assays.