Clinical and Radiographic Correlates of Primary and Reactivation Tuberculosis:  A Molecular Epidemiology Study FREE

Traditionally, active tuberculosis (TB) disease has been classified as either primary or secondary. Many researchers consider primary and secondary TB to reflect the time between the initial infection with Mycobacterium tuberculosis and the onset of clinical disease. In the literature, the exact interval that distinguishes primary from secondary TB ranges from 1 to 5 years.¹

Primary and secondary TB are also thought to have characteristic radiographic and clinical features: primary TB is said to be characterized by lower-lobe disease, adenopathy, and pleural effusions, and termed atypical, whereas secondary, or reactivation, TB is associated with upper lobe disease and cavitation, termed typical.^2- 5 These clinical observations, however, were based on studies conducted before the availability of molecular fingerprinting techniques and relied on often incomplete and circumstantial data. The Figure shows an example of the typical and atypical patterns in 2 of our study patients.

Pulmonary TB in the human immunodeficiency virus (HIV)/AIDS population is often characterized by adenopathy, mid or lower lung zone disease, effusions, and a paucity of cavitary lesions.^6- 8 Because persons with HIV/AIDS are prone to TB, some have attributed this atypical radiographic appearance to the susceptibility of HIV-infected patients to rapid progression from initial infection to active TB disease.⁹ Others have raised the question of whether the atypical radiograph results from altered immunity and may therefore also represent reactivation of long-standing latent infection in the setting of an abnormal host immune response.¹⁰

Molecular epidemiology studies of TB allow comparison of clinical and radiographic features of TB cases, which are likely to be related in time and space (ie, clustered isolates) with those that are not (unique isolates).^11- 12 In the present study, we use this approach to test whether recently transmitted cases have radiographic features distinct from distantly acquired infection and secondly, whether the atypical features of the radiograph in HIV-associated TB are due to a preponderance of recent infection or are manifestations of altered immunity in the reactivation of latent infection.

Study participants included all adult patients with culture-proven pulmonary TB at Columbia University Medical Center between 1990 and 1999 with pulmonary involvement, defined by at least 1 positive culture from sputum or from pleural effusion, and who also had available radiographic data. Demographic data were abstracted from the New York City Tuberculosis Control Program’s registry. This study was approved by the Institutional Review Board of the College of Physicians and Surgeons, Columbia University.

Restriction fragment length polymorphism (RFLP) analysis (DNA fingerprinting with the IS6110 insertion sequence) was performed at the Public Health Research Institute, Newark, NJ, according to internationally standardized methods.¹³ In summary, M tuberculosis DNA was extracted, digested with PvuII, subjected to electrophoresis, and hybridized by Southern blotting with a fragment of the insertion element IS6110. Identical strains recovered from 2 or more patients comprised a cluster of cases. Strains found in only 1 person were considered unique. M tuberculosis strains with fingerprints of 5 or fewer bands were subjected to secondary DNA analysis with spoligotyping (spacer oligonucleotide typing).^14- 16 These low-band RFLP patterns were assigned as clustered only if their spoligotypes were identical and specific to the IS6110 fingerprint. If they were not, they were classified as unique cases. Clustered cases were assumed to represent recent transmission, whereas a unique case was considered to result from the reactivation of latent disease.¹⁷

Data from the chest radiographs were obtained from review of reports dictated by attending radiologists at the time of admission and recorded in the electronic patient records. If multiple radiographs were available, the first radiograph performed on an admission for which a positive sputum culture was isolated was chosen for analysis. Radiographic findings were recorded as categorical variables and included the presence or absence of 6 features: upper lobe infiltrate cavitary lesion, hilar or paratracheal lymphadenopathy, middle or lower lobe infiltrates, pleural effusion, and miliary pattern.

The radiographs were considered typical if either an upper lobe infiltrate or a cavitary lesion in the upper lung zones was present. The presence of lymphadenopathy, a lower or middle lobe infiltrate, or effusion in conjunction did not change the characterization as typical. On the other hand, atypical radiographs were those with lymphadenopathy, lower or middle lobe infiltrates, or effusions without the presence of either a cavity or upper lobe infiltrate. Radiographs with a cavitary lesion in the middle or lower lung zones were considered atypical. Available radiographs were reviewed independently by 2 of the authors (J.B. and N.W.S.) and the κ statistic was calculated between each reader and the abstracted radiographic data from the chart.

We did not have access to information on treatment with antiretroviral drugs. We conducted a separate and parallel analysis using only data before 1997, ie, before the advent of widespread treatment with highly active antiretroviral treatment to see if its use coincided with changes in the observed relationships between clinical, molecular, and demographic predictors and radiographic appearance.

Associations between radiographic and clinical variables were tested with χ² statistical tests. Unknown values were excluded from univariate analysis. Continuous variables were made into categorical variables for univariate and multivariable analysis. Multivariable analysis was conducted with both logistic regression modeling and a generalized estimating equation model to adjust standard error estimates for potential correlation among clustered observations.¹⁸ Human immunodeficiency virus status was recorded as a 3-leveled categorical predictor with positive, negative, and unknown. Age was divided between those 60 years or younger and those older than 60 years. The age of 60 years was chosen as a cutoff because previous analysis involving this group of patients showed a precipitous drop in proportion of clustered patients older than 60 years.¹⁹ A significant drop in the risk of clustering occurred among patients in whom TB was diagnosed after 1993; therefore, the year of diagnosis was analyzed as a categorical variable (1990-1993 or 1994-1999). All analyses were conducted using SAS version 9.1 (SAS Institute Inc, Cary, NC). P<.05 was considered statistically significant.

Race and ethnicity classification was based on patient self-report, as recorded in the New York City Department of Health Tuberculosis Control Program’s case registry. This variable was included in our analyses because of prior suggestions that racial groups differ in susceptibility to TB and could have therefore influenced findings.

There were 546 culture-proven adult cases of TB at Columbia University Medical Center between 1990 and 1999 with corresponding demographic data available at the Department of Health, Tuberculosis Control Program. Of these, 484 (89%) had pulmonary involvement and among these 456 (94%) had radiographic data available in the computerized charts.

Columbia University Medical Center serves Washington Heights and is adjacent to Harlem. Washington Heights has served as a destination for foreign-born persons from the Caribbean for a long time and the average time in the United States among our foreign-born patients was 14.1 years with a median of 14 years. In Harlem most patients are African American and US born. A total of 35.8% of our study population was foreign-born, and 54.3% with known HIV status were HIV infected. Most were men (69.5%) and 83.6% were younger than 60 years. About half the TB isolates from these patients (51.8%) were part of molecular epidemiologic clusters. The mean age of the population was 41 years with a median of 38 years. Characteristics of the patient population are shown in Table 1. Because RFLP data were available for 546 patients, clustering data were assigned based on the original number of patients. There were 51 separate clusters with mean size of 5.2 and a median of 3.

Overall 276 (60.5%) had typical radiographs while 180 (39.5%) had atypical radiographs. Cavitary lesions were present in 28.7%, upper lobe infiltrates in 58.3%, and lymphadenopathy in 22.6% of the patients. The radiographic characteristics are shown in Table 2. Association between radiographic predictors was assessed using a binomial test whereby each paired observation is assigned a number 1 for concordance and 0 for discordance and the sum is compared with a normal distribution. Fifty-four physical radiographs were read by 2 authors (J.B. and N.W.S.). The κ statistic between radiology reading and N.W.S. was 0.799; between radiology reading and J.B., 0.761; and between N.W.S. and J.B., 0.683.

Clustering was significantly associated in an inverse manner to the presence of an upper lobe infiltrate with an odds ratio (OR) of 0.68 and a 95% confidence interval (CI) of 0.47-0.99, as well as with a typical radiograph (OR, 0.58; 95% CI, 0.39-0.84). Clustering was not significantly related to any other radiographic feature. There were 26 patients (5.7%) whose radiographs had only effusions, and the presence of only an effusion was not significantly associated with a clustered strain (OR, 0.56; 95% CI, 0.25-1.27). Clustered isolates were not significantly associated with the presence of a cavitary lesion on the radiograph (OR, 0.85; 95% CI, 0.56-1.27).

When the relationship between cluster status was stratified by HIV status, several strong and significant associations became evident. In both groups of patients with clustered and unique strains, HIV infection was associated with fewer cavitary lesions, fewer upper lobe infiltrates, more lymphadenopathy, and fewer typical patterned radiographs. No significant interaction between HIV and cluster status was found: the Breslow-Day tests for heterogeneity between HIV status and radiographic outcome across strata of cluster status was not statistically significant in any case. These results are shown in Table 3.

Univariate analysis of characteristics associated with typical radiographs found the following associations: any resistance (OR, 2.11; 95% CI, 1.00-4.47), clustered RFLP (OR, 0.68; 95% CI, 0.47-0.99), HIV status (OR, 0.21; 95% CI, 0.13-0.34). Age older than 60 years (OR, 1.64; 95% CI, 0.97-2.78), and non–US birth (OR, 1.42; 95% CI, 0.96-2.11) were nearly significantly associated with typical radiographs. Analysis of the relationship between demographic and social predictors across strata of HIV showed no significant interaction as tested by the Breslow-Day test for heterogeneity (Table 4). 

Generalized estimating equations were used to model the relationship between significant predictors in univariate analysis with radiographic appearance in multivariable analysis to provide a more conservative approach to estimates of the SE in the variable cluster status based on the number of clusters (n=51) rather than the number of clustered observations. The model found that HIV was the most significant predictor of radiographic appearance with an OR of 0.20 (95% CI, 0.13-0.31) for association between HIV infection and typical radiograph. Clustering, age, and foreign birth were also entered into the model but were not significantly associated with radiographic appearance. Any resistance remained significant (OR, 3.02; 95% CI, 1.34-6.78). These results are shown in Table 5.

We repeated the analysis using only the data gathered before 1997, when the use of highly active antiretroviral treatment came into widespread use, to see if its use coincided with significant changes in the data. There were no changes in the outcome, and again HIV status and any resistance were the only significant predictors of radiographic appearance.

We demonstrate in a large series of epidemiologically and clinically well-defined patients with TB that the most significant independent predictor of radiographic appearance is HIV status. Cluster status, which allows us to distinguish recently acquired from remotely acquired TB, is not a significant predictor of radiographic appearance.

A recent study also found no difference in radiographic presentations between primary and reactivation disease.²⁰ Our study, larger and with more statistical power, confirms and strengthens those findings. The current study involved 456 patients and was powered at 95% to detect a difference of 15% between study populations, which makes us reasonably confident that we would have been able to detect such an association if one existed.

Our findings are in conflict with older literature showing reactivation of infection acquired long ago to be manifest as upper lobe infiltrates and cavitary lesions and recently acquired disease to produce lymphadenopathy, effusions, and lower and middle lung zone infiltrates. These studies, however, were conducted and published before molecular techniques were available and were limited by unreliable definitions of recently acquired disease that included both valid measures, such as documented purified protein derivative conversion to more suspect historical and even tautological radiographic findings. Choyke et al² in 1983 reported the radiographic features of primary pulmonary TB in a case series, but only 64% had documented purified protein derivative conversions, whereas the other patients were classified based on radiographic features, such as the presence of adenopathy or pleural effusions or clinical criteria. Woodring et al⁴ found less cavitation and fewer upper lobe infiltrates among patients with primary TB, but inclusion criteria did not require culture-proven diagnoses, and nearly half of the primary cases were children.

The highly significant and strong association between HIV status and atypical radiographs argue that immune status is the major determinant of atypical radiographs in HIV patients and imply strongly that time had elapsed between acquisition of infection and development of active disease is not a strong association. Outbreak investigations have shown that HIV patients are susceptible to infection and clinical disease shortly thereafter,²¹ and a consequent theory has been that the atypical radiograph of HIV-associated TB is due to preponderance of primary disease. Our findings argue against this theory. In fact, our results show that HIV infection predicts atypical radiographs in both primary disease and reactivation, that is, among clustered and nonclustered cases.

In our study and in a previously reported study, pleural TB was associated with clustering 35% to 40% of the time.²² We found, however, that as was true with radiographic presentations overall, pleural effusions were more likely to reflect underlying HIV infection than molecular epidemiologic linkages. The absence of an association in our study between the presence of an effusion and clustering, in contrast to earlier studies, may perhaps be explained by the fact that 53.4% of patients with known HIV status were HIV-positive.

The association of drug-resistant isolates with typical radiographic findings was interesting because it is not immediately apparent why drug susceptibility or resistance should affect radiographic features of TB. We speculate that this finding may be explained by considering that the manifestations of active TB result from a balance between the host immune response and mycobacterial virulence. Both laboratory and clinical studies have suggested that drug-resistant organisms are less virulent.^23- 24 It seems plausible that either weakened host immune responses or more virulent pathogens could result in atypical presentations and, conversely, that less virulent pathogens may predispose to more typical presentations. This is consistent with our finding that HIV-infected individuals have atypical radiographic findings and could explain why drug-resistant but less virulent strains appear more typical.

There are limitations to our study. Each cluster necessarily includes a case of reactivation TB, we were unable to determine which member of a cluster that would be and hence which radiograph would be misclassified.²⁵ This bias, however, would tend to bias the results toward the null so significant findings are likely to remain valid. Furthermore, our study population is derived from a single academic medical center, and hence may be susceptible to selection bias. Our patients included a high percentage of HIV patients. Although this may distinguish our patients from those in other studies, we sought to test hypotheses about the pathogenesis of TB rather than the epidemiology, and hence the results may be generalizable to different populations. That nearly half of our patients with known HIV status were HIV-infected maximizes the power of the comparison between HIV-infected and HIV-uninfected groups.

In the past 10 years, fingerprinting of M tuberculosis has led to numerous important insights into the epidemiology and program management of TB that have significantly changed the way we understand this disease.²⁶ This study applies this technique to advancement of our understanding of fundamental features of M tuberculosis infection and pathogenesis and reiterates the utility of fingerprinting in TB research.

In summary our findings argue that the terms primary and reactivation TB are misleading when used to make inferences linking radiographic findings to epidemiologic characteristics of patients. Radiographic findings have implications regarding host immune status of patients, but whether a patient’s disease is due to recently transmitted or remotely acquired infection cannot be determined from them.