In this study, we serially tested 18 HIV-1-infected women who were IGRA-positive at baseline during pregnancy, at 3 monthly intervals during the first-year postpartum to determine long-term within-person consistency of IGRA responses and to describe the effect of the peripartum period on IGRA responses. We observed that 83% of women had a subsequent positive response and 50% retained a positive IGRA response at all subsequent assays throughout the postpartum period. Women with higher magnitude of combined response at baseline were more likely to have consistently positive responses at subsequent time points. Women with weak positive response close to the cut-off value at baseline had subsequently fluctuating IGRA responses. The magnitude of the combined IFN-γ response and antigen-specific responses increased slightly during the 1-year postpartum period.
Our study was conducted in a setting with high TB incidence in HIV-1-infected women with high probability of TB exposure but no specific known TB exposure. We found that HIV-1-infected pregnant women with latent TB infection had generally reproducible positive IGRA responses. Although 2011 WHO guidelines recommend universal isoniazid preventive therapy (IPT) in HIV-1-infected individuals, IGRA-targeted IPT may be a strategy to consider for pregnant women [19
]. Consistent detection of latent TB by IGRAs despite physiologic changes during/after pregnancy would strengthen specificity of this approach.
While most women had repeated positive IGRAs in our study, we observed some reversions and noted within-person variability of responses, which could be due to changes in TB exposure, TB immune responses, or test reproducibility. Our observation of fluctuation in responses is consistent with previous serial IGRA studies, in which conversions and reversions were observed more frequently in individuals with responses close to the predefined cut-point for a positive response [20
]. One South African study with close repeat testing (2 days apart) of 15 health care workers (HCWs) noted 100% concordance in results [23
]. However, in TB-exposed HCWs in India with serial IGRAs, 2/14 (14%) had a change in QFT responses with magnitude of IFN-γ
declining 12 days after baseline assessment [21
]. Another study in HCWs in South Africa using both QFT and T-SPOT.TB at 4 times over 21 days observed that T-SPOT responses were more likely to change (revert/convert) than QFT and noted considerable within-person variability of responses [20
]. Finally, among HCWs in Germany, a low TB incidence country, there were more reversions than conversions during serial testing and age and prior positive TST predicted consistent QFT positivity [22
In our study, women with baseline response of >8 SFCs/well were 10 times more likely to have consistently positive IGRAs postpartum while those with lower levels had less consistent responses. These data are consistent with previous studies noting that responses close to the cut-point are more likely to fluctuate [20
]. The US FDA and Oxford Immunotec recommend a borderline “grey zone” of 5–7 spots above negative control for the T-SPOT.TB and suggest that results be considered in conjunction with clinical information or to retest [16
]. We also observed a substantial within person variability in the quantitative responses, which probably accounts for much of the fluctuation in the qualitative responses between positive and negative. These findings underscore the importance of considering quantitative IFN-γ
response data in addition to the dichotomous (positive/negative) results in serial testing and in clinical decisions.
Our study has important strengths and limitations. Our study is the first to describe long-term within-person reproducibility of T-SPOT.TB IGRA in HIV-1-infected women during the pregnancy/postpartum period in a TB-endemic setting in the absence of any specific exposure to TB. The women included in this study had longitudinally measured clinical and immunological outcomes, enabling us to take these factors into account since they could potentially alter IGRA responses. We were also able to repeat the IGRAs for these women at relatively close intervals and at critical time points such as early postpartum and during breastfeeding. Our study involves women without a defined time of TB exposure or LTBI treatment. This enabled us to describe the natural variation in IFN-γ
responses during the pregnancy/postpartum period. In contrast to previous studies on serial IGRAs, which have been typically shorter, our period of serial evaluation was longer (over 1 year). A limitation of our study was the use of cryopreserved PBMCs as opposed to fresh samples, which might have contributed to test variability [24
]. In a previous study of T-SPOT.TB results comparing fresh and frozen PBMCs, cryopreserved samples had lower sensitivity [25
]. In our study, lower sensitivity due to cryopreserved PBMCs may have contributed to greater fluctuation of results between positive and negative. However, a new serial assessment study using fresh assays would not be feasible because a positive test would indicate LTBI treatment based on new guidelines. In addition, a few published studies have noted adequacy of cryopreserved samples for T.SPOT.TB and other ELISpot assays [26
]. A design constraint of studying LTBI responses in this setting is that women are frequently exposed to TB. Thus, the alternative design including women who were IGRA negative at baseline and followed for consistency of “negative” responses would be confounded by women with new positive IGRAs who newly acquired LTBI. We therefore restricted the study to those with baseline positive IGRAs to assess consistency. Selective inclusion of baseline positives would be expected to bias estimates of rate of change in IFN-γ
response. To circumvent this potential bias, we evaluated changes after the baseline visit. Our sample size, though small, was comparable or larger than previous serial studies on IGRAs.
In conclusion, our study demonstrates consistency of IGRAs during pregnancy and postpartum in HIV-1-infected women, particularly among those with higher magnitude responses. Fluctuation in responses between positive and negative was seen among women with weak positive responses at baseline. Despite hormonal and immune perturbations during the postpartum period, the magnitude of response did not change markedly over the postpartum period, as shown by the results from the linear mixed models. The slight increase in levels over time during the postpartum period may reflect some impact of immunosuppression during pregnancy, which may explain increased susceptibility to active TB during this period. The burden of TB in HIV-1-infected women in the childbearing years and the consequent risk of TB morbidity and mortality in their infants is well established [1
]. Further serial studies in this setting will be useful to define optimal timing for IGRA testing during or after pregnancy and to understand biologic determinants of IGRA responses and magnitude.