^{1}

^{1}

^{2}

^{*}

Objective: The aim of this retrospective study was to evaluate diagnostic accuracy of serum thyroglobulin antibody (TgAb) in thyroglobulin (Tg)-negative and TgAb-positive (Tg
^{-}
TgAb
^{+}
) patients with differentiated thyroid carcinoma (DTC). Method: We studied 341 patients with histologically confirmed DTC who had undergone remnant ablation and showed Tg
^{-}
TgAb
^{+}
assessed by electrochemiluminescence immunoassay (ECLIA). The cases were divided into two groups, including recurrence group 119 cases and no evidence of disease (NED) group 222 cases. Receiver operating characteristic (ROC) curve analysis was carried out. The symmetric measures of the two diagnostic methods (the golden standard and the diagnostic standard as serum TgAb level alone) were analyzed using McNemar test and measure of agreement Kappa. Results: Serum TgAb level (1381.292 ±
1017.221) IU/ml of patients with recurren
t
group was significantly higher than that (125.559
±
314.047) IU/ml of NED group (P = 0.000 <
0.001).
The area under the ROC curve was 0.962 and its asymptotic 95% confidence interval (CI) was (0.942
,
0.982) that was high statistical significance. The cut-off value of TgAb was determined and interpreted at 246.695
IU/ml with sensitivity (92.40%) and specificity (92.30%). McNemar test showed that the diagnostic results of the two methods were not significant difference (P = 0.230 >
0.05). Measure of agreement Kappa was 0.841, P
=
0.000
<
0.001 that showed the agreement of the two diagnostic methods was significan
t
. Conclusion: Serum TgAb is a useful tumor marker for recurrence in Tg-negative and TgAb-positive DTC patients who underwent thyroidectomy and remnant ablation. The cut-off value of TgAb is 246.695 IU/ml, that is to say, serum TgAb level upon 246.695 IU/ml may be associated with the persistence or recurrence of DTC.

Serum thyroglobulin (Tg) level and ^{131}I whole-body scanning (WBS) are the leading recognized sensitive and specific tools for the detection of recurrence or metastases during follow-up of thyroidectomized patients with differentiated thyroid carcinoma (DTC) who underwent remnant ablation. These have been widely accepted management protocols [^{+}) in DTC patients ranges from 10% to 30% [^{+} DTC patients. TgAb currently compromises the use of serum Tg as a tumor marker in this kind of DTC patients. Also, TgAb has been studied extensively in order to evaluate its place in clinical practice [^{−}TgAb^{+}) DTC patients.

Therefore, the aims of the present study were to evaluate diagnostic accuracy of serum TgAb in Tg^{−}TgAb^{+} DTC patients who had previously received thyroidectomy and radioiodine ablation therapy and ascertain the cut-off value of TgAb to differentiate between the pathologic and disease-free patients.

We performed a retrospective review on medical records of1236 patients with histologically confirmed DTC who were admitted to the Nuclear Medicine Department of Zhongshan Hospital Xiamen University from January 2001 to December 2014. All patients underwent total or subtotal thyroidectomy, followed by radioiodine remnant ablation completely and thyroid hormone suppression of thyroid-stimulating hormone (TSH).

According to the collecting and excluding standards, 341 patients entered the study finally. The patients were divided into the recurrence group (n = 119) and no evidence of disease (NED) group (n = 222) on the basis of the golden standard.

A serum TG level of below 1 µg/L with suppressed TSH levels or 2 µg/L with stimulated TSH levels was considered undetectable (Tg^{−}). We defined TgAb-detectable (TgAb^{+}) as upon 10 IU/ml. Recurrence was defined as either a regional recurrence and/or distant spread.

The DTC patients with Tg^{−}TgAb^{+} and successful ablation of thyroid remnant en- rolled the study. The DTC patients with TG-detectable, TgAb-undetectable (Tg^{+}TgAb^{−}) or thyroid remnant in thyroid bed were excluded the study. The gold standard for the study was either the diagnosis of recurrence established by biopsy and/or the diagnosis of the routine clinical examinations (chest x-rays, CT scanning, neck high-resolution echography and radioiodine whole body scan).

Sensitivity (Sen) and specificity (Spe) were defined as true positive and true negative, respectively. Positive likelihood ratio (+LR) was defined as the ratio of the true positive and false positive (Sen/1 ? Spe). Youden Index (YI) = Sen + Spe ? 1. Pre-test probability is defined as the probability that a patient suffers from recurrence or metastases by means of his/her medical history and physical sign. Post-test probability is defined as the probability of recurrent disease in a patient with a certain test result of TgAb. Pre- test odds = pre-test probability/(1 ? pre-test probability). Post-test odds = pre-test odds × (+LR). Post-test probability = post-test odds/(1 + post-test odds).

Serum Tg level was determined by means of electrochemiluminescence immunoassay (ECLIA) method using a commercially available kit (Roche Diagnostics GmbH), which has a measuring range of 0.100 - 1000 µg/L. The intra- and interassay coefficient of variability (CV) were 1.8% and 3.2%, respectively.

Serum TgAb level was measured by means of ECLIA method (Roche Diagnostics GmbH) with measurable range of 10 - 4000 IU/ml. The intra- and interassay CV were 5.2% and 7.3%, respectively.

Data were expressed as mean ± SD. Statistical analysis was done using Independent-samples t test and the χ^{2} test. A P-value of less than 0.05 was considered to indicate a statistically significant difference.

The analysis of diagnostic value was performed by receiver operating characteristic (ROC) curve at the time of evaluation. The nonparametric method could be used to estimate the area under ROC curve. The area under the ROC curve (AUC) with 95% confidence interval (CI) was calculated to express the overall diagnostic accuracy of the test. The AUC, P value, and cutoff point were obtained from the curve. The cut-off value was defined as the threshold value of the maximum Youden Index (YI). Corresponding to Spe, Sen, PV and LR were calculated for the cut-off value in contingency tables.

The symmetric measures of the two diagnostic methods (the golden standard and the diagnostic standard as serum TgAb level alone)were analyzed using McNemar test and measure of agreement Kappa. And, the analysis of positive likelihood ratio (+LR) with different threshold values were carried out in the study.

Statistical analyses were performed with SPSS for Windows Software package (Release 17.0, SPSS Inc., US).

A total of 341 patients were enrolled and divided into two groups: 119 patients with recurrence group and 222 patients with NED group between January 2001 and December 2014.

Characteristic | Recurrence Group | NED Group |
---|---|---|

Patients―no. (%) | 119 (34.897) | 222 (65.103) |

Sex―no.*^{††} | ||

Female | 103 | 177 |

Male | 16 | 45 |

Age―years | ||

Mean ± SD^{†§} | 47.64 ± 11.96 | 45.74 ± 11.25 |

Range | 16 - 82 | 13 - 76 |

Histology―no.^{‡††} | ||

Papillary | 79 | 172 |

Follicular | 25 | 34 |

Papillary/Follicular | 15 | 16 |

Serum TgAb level (IU/ml) | ||

Mean ± SD^{¶}^{§} | 1381.292 ± 1017.221 | 125.559 ± 314.047 |

Median | 1038.400 | 63.745 |

Range | 53.230 - 4000.000 | 10.000 - 4000.000 |

*There was not statistically significant difference (χ^{2} = 2.457, P = 0.117 > 0.05). ^{†}No significant difference was found (t = 1.454, P = 0.147 > 0.05). ^{‡}It was not statistically significant difference (χ^{2} = 5.229, P = 0.073 > 0.05). ^{¶}Serum thyroglobulin autoantibodies (TgAb) levels were significantly higher in the recurrence group as compared with that in the NED group (t = 13.135, P = 0.000 < 0.001). ^{§}Comparison of Recurrence Group vs. NED Group by Independent-Samples T Test. ^{††}Comparison of Recurrence Group vs. NED Group by χ^{2} test.

from 0.5 (P = 0.000 < 0.001). The 95% CI for the area was (0.942, 0.982). The interval did not consist of 0.5.The results showed that serum TgAb level reflects the recurrence of DTC in Tg^{−}TgAb^{+} patients after thyroid ablation successfully. Point A on the curve is the closest to the upper left corner. Its ordinate is 0.924, its abscissa is 0.077. If point A is defined as optimal operating point, corresponding to Sen and Spe are 92.40%, 92.30%, respectively.

YI is an index of analysis by synthesis that reflects the authenticity of diagnostic test. It is defined as (Sen + Spe ? 1); its range is from −1 to +1. As the analysis of definite quantity, the closer the value is to the upper limit (+1), the higher the overall accuracy of the diagnostic test.

We selected the operating point on the ROC curve the closest to the upper left corner as cut-off point (point A showed

The symmetric measures of the two diagnostic methods (the golden standard and the diagnostic standard as serum TgAb level alone) were analyzed using McNemar test and measure of agreement Kappa. McNemar test showed that the diagnostic result of the two methods was not significant difference (P = 0.230 > 0.05). Agreement Kappa (κ) is a measurement of evaluating agreement of the diagnostic result of the two methods. Thus, κ ≥ 0.7 is considered to be excellent test results (i.e., the agreement of two diagnostic methods was high statistical significance); 0.4 ≤ κ < 0.7 is considered to be statistical significance; κ < 0.4 is considered to be no statistical significance. Measure results of agreement Kappa was “κ = 0.841, P = 0.000 < 0.001”, that showed the agreement of the two diagnostic methods was high statistical significance.

Selecting TgAb = 246.695 IU/ml as cut-off value, we predicted the studied patients (

The analysis of +LR with different threshold values were calculated in the study (

Clinical routine diagnostic methods (Golden standard) | Total (n) | ||
---|---|---|---|

Recurrence (n) | No evidence of disease (n) | ||

TgAb ≥ 246.695 (n) | 110 | 16 | 126 |

TgAb < 246.695 (n) | 9 | 206 | 215 |

Total (n) | 119 | 222 | 341 |

TgAb (IU/ml) | Recurrence group | NED group | +LR | ||
---|---|---|---|---|---|

n | Ratio | n | Ratio | ||

≥1000.000 | 62 | 62/119 = 0.521 | 5 | 5/222 = 0.023 | 0.521/0.023 = 22.652 |

246.695 ≤ TgAb < 1000.000 | 48 | 48/119 = 0.403 | 12 | 12/222 = 0.054 | 0.403/0.054 = 7.463 |

100.000 ≤ TgAb < 246.695 | 6 | 6/119 = 0.050 | 42 | 42/222 = 0.189 | 0.050/0.189 = 0.265 |

10.000 ≤ TgAb < 100.000 | 3 | 3/119 = 0.025 | 163 | 163/222 = 0.734 | 0.025/0.734 = 0.034 |

Total | 119 | 1 | 222 | 1 |

246.695 IU/ml, 29.109 times that of 10.000 IU/ml ≤ TgAb < 100.000 IU/ml; the possibility of recurrence as 246.695 IU/ml ≤ TgAb ≤ 1000.000 IU/ml was 4.209 times that of 100.000 IU/ml ≤ TgAb < 246.695 IU/ml, 26.802 times that of 10.000 IU/ml ≤ TgAb < 100.000IU/ml; the possibility of recurrence as 100.000 IU/ml ≤ TgAb < 246.695 IU/ml was 6.368 times that of 10.000 IU/ml ≤ TgAb < 100.000 IU/ml.

According to the analysis of positive likelihood ratio, we assumed that the possibility of recurrence/metastases as 246.695 IU/ml ≤ TgAb < 1000.000 IU/ml is 10 units, the possibility of recurrence/metastases as TgAb > 1000.000 IU/ml, 100.000 IU/ml ≤ TgAb < 246.695 IU/ml and 10.000 IU/ml ≤ TgAb < 100.000 IU/ml is 10.860 units, 2.376 units, 0.373 units respectively (

Self antigen of TgAb is TG. And, TG is incomplete “inaccessible antigen” and giant molecule glycoprotein synthesized by thyroid follicular epithelial cell. Serum TgAb is usually of IgG1, IgG3 or IgG4 subtypes in DTC patients [^{−}TgAb^{+} DTC patients. Some studies [^{−}TgAb^{+} DTC patients. That is, these kinds of patients were additionally diagnosed as recurrent or metastatic disease by measuring their serum TgAb levels. Some studies [

persistent tumors mainly become source of antigenic components of producing TgAb. The notion theoretically explains that serum TgAb level is an index of recurrence or metastases in Tg^{−}TgAb^{+} DTC patients of complete ablation.

In this study, we analyzed the diagnostic accuracy and ascertained the cut-off value of serum TgAb in the enrolled patients by means of ROC curve and +LR analysis of different threshold values. Serum TgAb levels (1381.292 ± 1017.221) IU/ml of patients with recurrence group was significantly higher than those (125.559 ± 314.047) IU/ml of NED group (P = 0.000) (^{−}TgAb^{+}. The result showed serum TgAb can indicate active tumor. Research results of most scholars [^{−}TgAb^{+}. These notions provide practically reasons. But, it is controversial what its cut-off value of serum TgAb is. Only according to their clinical experience, most scholars [

According to our knowledge, this is the rare report on the application of ROC curve and +LR with different threshold values analyses in order to evaluate the diagnostic value of serum TgAb monitoring in the follow-up after successful ablation remnant thyroid tissue for TG-undetectable (TG^{−}) DTC patients. ROC curve is applied to evaluate the accuracy of diagnostic test. ROC curve occupy a central or unifying position in the process of assessing and using diagnostic tools. The ROC curve depicts the overlap between the two distributions by plotting the Sen vs. (1 − Spe) for the complete range of decision thresholds. The y-axis is Sen, and the x-axis is (1 − Spe). The ROC graph is a plot of all of the Sen/Spe pairs resulting from continuously varying the decision threshold over the entire range of results observed. Every point on the curve re- presents a Sen/Spe pair corresponding to a particular decision threshold [

AUC is a measure of predicted accuracy. The results of our ROC study showed that AUC value was 0.956 (_{z} (0.5). And the AUC value (0.956) was higher than 0.9. The results showed that the diagnostic test was higher accuracy. That is to say, it was important diagnostic significance for serum TgAb level to evaluate recurrence in DTC patients of TG^{−}TgAb^{+}. Even, it may be possible that TgAb is more likely to act as a tumor marker in these patients.

It showed that TgAb at a single cut-off level was used with regard to monitoring the recurrence of the DTC patients with TG^{−}TgAb^{+} received successful ablation. As showed in

In the present study, the analytical results of +LR with different threshold values showed that the higher serum TgAb level, the more possible recurrence is (

Hjiyiannakis et al. [^{+} in the Royal Marsden Hospital. They reported that 11 patients were found recurrence among 28 patients of high titre group (TgAb > 1/100); only 2 patients were found recurrence among 12 patients of low titre group (TgAb < 1/100). Median of TgAb level was 1/604 in the high titre group, whereas it was 80 IU/ml in the low tire group. In our study, median of TgAb level was 1038.400 IU/ml in the recurrence group, and is similar to the observations of Hjiyiannakis et al. [

Serum TgAb is acted as a “tumor marker” and its cut-off value is 246.695 IU/ml in TG^{−}TgAb^{+} DTC patients with successful ablation of remnant thyroid tissue. The higher serum TgAb level, the more possible recurrence is. During the long follow-up of such patients populations, serum TgAb level should be routinely measured. As serum “TgAb ≥ 246.695 IU/ml”, correlative clinical measures should be carried out in order to find recurrence earlier and treating as soon as possible; as serum “TgAb < 246.695 IU/ml”, we should termly monitor the serum level of TgAb.

We need carry out a research of big sample and a long-term clinical observation to nail down the diagnostic value of serum TgAb in patients with DTC.

This study was supported by a grant from the National Natural Science Foundation of China (NSFC) (81071182).

Fan, Q., Su, X.H. and Kuang, A.R. (2016) The Diagnostic Value of Serum TgAb in the Tg-Negative and TgAb-Positive DTC Patients after Successful Ablation. Journal of Cancer Therapy, 7, 889-900. http://dx.doi.org/10.4236/jct.2016.712086