Results
A total of 174 patients with COVID-19 required ABG sampling in our
hospital wards during the study period. After the exclusion of 57
patients, a total of 117 patients’ data was evaluated (Figure 1). The
mean age of study patients was 69.4 ± 12.0 years, and 65% (n:76) were
male. Of all study patients, 97 (82.9%) had at least one comorbid
disease. The computed tomography findings of thorax were compatible with
COVID-19 pneumonia in 110 (94%) patients. The patients were
hospitalized at the median 4th [2-6] day, and the ABG samples were
analyzed at the median 11th [8-15] day after symptom onset. Among
the patients, 98.3% received anticoagulant, and 45.3% received
antiaggregant agents during hospitalization.
Twenty-nine (24.8%) patients were transferred to the intensive care
units and 14 (12.0%) died.
The median SpO2 and SaO2 levels of the
patients were 88% [84-88] and 91.8% [88.3-94.4], respectively.
In 10 out of 117 patients, SpO2 levels were higher than
SaO2 (mean difference 1.1±0.7%). We categorized the
patients into two groups; in whom the difference between
SaO2 and SpO2 was ≤4% (acceptable
difference group) and >4% (large difference group). In 59
patients (50.4%), the difference between SpO2 and
SaO2 measurements was greater than 4% (large
difference), and within this group, all SaO2 levels
measured were higher than SpO2. The baseline features
and comorbid conditions of these two groups were given in Table 1.
Patients with large difference have higher neutrophil count d-dimer,
ferritin, fibrinogen and C-reactive protein (CRP) level than the
patients with an acceptable difference (Table 2). To determine the
effect of clinical and laboratory parameters on large difference risk, a
binary logistic regression analysis was employed; revealing that
increased d-dimer, fibrinogen, ferritin level and decreased lymphocyte
count were significantly associated with large difference between
SpO2 & SaO2 (Table 3).
We performed receiver operating characteristic (ROC) curve analyses to
determine cut-off values for ferritin, fibrinogen and lymphocyte count
that would predict large difference between SpO2 and
SaO2. The best cut-off value was 4.8 g/dL (area under
curve-AUC: 0.761, 95% CI: 0.674–0.848, p < 0.001,
sensitivity:71%; specificity:73%) for fibrinogen, 228 g/dL (AUC:
0.813, 95% CI: 0.734–0.892, p < 0.001, sensitivity:86%;
specificity:57%) for ferritin, and 1,04 x
103/mm3 (AUC: 0.806, 95% CI:
0.722–0.890, p < 0.001, sensitivity:86%; specificity:70%)
for lymphocyte count.
Bland-Altman analysis comparing SpO2 with
SaO2 within the total study group demonstrated the
negative bias (mean difference) of 4.02% with an SD of 2.65 (precision)
and the limits of agreement of −9.22% to 1.17% (Figure 2). This
indicates that the SpO2 underestimated
SaO2 value by an average 4.02 ± 2.65% and limits of
agreement were clinically important since SpO2 could be
measured 9% lower or 1% greater than actual arterial oxygen
saturation. Also, we performed Bland-Altman analyses to calculate mean
differences and precision separately in subgroups of patients with
increased and normal serum ferritin, with increased and normal serum
fibrinogen; normal and decreased lymphocyte count. The results showed a
significant increase of the bias in patients with high fibrinogen level,
high ferritin level and low lymphocyte count compared those with normal
values (Figure 3, Figure 4 and Figure 5, respectively). Within all
subgroups, we showed that SpO2 underestimated
SaO2. These finding suggested that the reliability of
pulse oximeter is further reduced in patients with high fibrinogen, high
ferritin and low lymphocyte count.